Pure, natural henna, cassia & indigo can be used over previously synthetically dyed hair. (as always, do a test with hair collected from your hairbrush first. If that's ok, do a small test section of your hair before henna'ing your whole head.

The reason hair stylists are told to never henna hair that's been synthetically dyed is because they are only taught about crappy, cheap boxed henna products in stores that have metallic salts added.

Be aware:
If your hair has been bleached & you want to henna: bleaching hardens the keratin of the hair, making it harder for the natural dyes to bind to the hair. It can also make the natural cassia / henna look brassy.

Always test first!

That depends on the ambient temperature & the pH of the liquid.
The lower the pH (more acidic), the longer it takes, but it's a better dye release.
The hotter the environment, the faster the dye release.
Ex: low pH Lemon juice in winter in a cool house can take 24 - 30 hours. Lemon juice on a super hot summer day can take 4 - 6 hours.
Ex: pH neutral water in winter - 6 hours. Water on a hot summer day = 2 hours.
(water is ok for body art, but never use water to dye release for hair dye: it will wash out).
This is why I always encourage testing instead of going by a specific time.
How to test:
Did you put paper towel over the plastic wrap? If so, is it stained orange? (the dye will transfer through the plastic wrap).
It's ready to test.
Test: dip some henna paste out that's under the surface (not the top brown, but the lower green), dab it on your palm for 5 minutes. Wash off. Is the stain bright orange? Then you're henna is ready. If not bright orange, test again in 1 - 2 hours.

1 - Lesser quality brand instructions were followed instead of Henna Canada instructions.

2 - henna was dye released with water or another neutral / alkaline liquid

3 - Mineral, sebum & product buildup on the hair. (Do a chelating treatment
& deep clean with a clarifying shampoo before dyeing)

4 - henna wasn't fully dye released

5 - henna paste sat too long, past dye release & was already demising before application

6 - A “moisturizing” or “conditioning” shampoo was used, or a shampoo bar or conditioner was applied before henna/indigo application

7 - Something else was added to the paste (egg, yogurt, oil )

8 -Hair wasn’t covered air-tight with plastic wrap it shower cap

9 - Paste wasn’t left on long enough

10 - Product with coconut oil was used after rinsing out the paste (shampoo, conditioner, oil product)

11 - You simply have dye resistant grey hair & need to use the tips & tricks for that problem

You used henna + indigo, but the indigo washed out.
Reason #1
Lesser quality brand instructions were followed.

REVIEW: How to use Indigo properly:
- use hot water (not boiling, but like a hot shower). Purified water if you can (no tap chlorinated).
- mix it into a thick paste and *immediately* combine with your henna and get it into your hair.
INDIGO IS USELESS AS SOON AS IT OXIDIZES - ie: turns from green to blue.

Other Common Reasons:
- Indigo powder was frozen (stored in freezer or was left out in the mailbox for too long in winter)
-Mineral, sebum & product buildup on the hair. (Do a chelating treatment
& deep clean with a clarifying shampoo before dyeing)
-The indigo paste was mixed before ready to use or it sat to “dye release”
- A “moisturizing” or “conditioning” shampoo was used, or a shampoo bar or conditioner was applied before henna/indigo application
- Something else was added to the paste (egg, yogurt, oil)
- Henna paste was too acidic before indigo was added
-Hair wasn’t covered air-tight with plastic wrap it shower cap
- Paste wasn’t left on long enough
- Product with coconut oil was used after rinsing out the paste (shampoo, conditioner, oil product)
- You simply have dye resistant grey hair & need to use the tips & tricks for that problem

Gray hair grows faster than pigmented hair, so it requires henna'ing more often.
Gray hair has harder keratin than pigmented hair, so it can resist dye.

First, double check that you're dye-releasing & mixing the paste properly. It might be covering grey for other reasons.

Tip 1: after washing your hair, do a baking soda mask on your grey hair areas. (mix baking soda with enough water to make a paste). Leave it on (covered with a cap to stay war, & moist) for 20 minutes. Rinse, then move onto henna'ing.
The high pH of baking soda will lift your hair cuticles which is great for dye uptake, but terrible for the feel of your hair. Doing a lemon-water rinse after you're all finished & a hot oil treatment the day after will help smooth your hair cuticles back down.

Tip 2: Add 1 tsp of salt for every 100g indigo powder you're using in your mix

Tip 3: Add heat. When the paste is in your hair & your head is covered, airtight, with a plastic cap or wrap, blast your head with heat from a hairdryer for 15 minutes.

Well, usually that just means that you have really healthy hair. The cuticles are strong & lay tightly together.
it could be that your cuticles are so tight that you have what's called "non-porous" hair.
In these cases, the reason hair takes up natural dye better on the old length than on the roots is because hair gets worn & roughed up with time.

What to do?
Use the same tips as for dye-resistant gray hair.

If you have long hair & you don't like the bottom being darker than the top, then you can henna just the first few inches of growth.

The hair cuticles have been a little raised & roughed up from the acidic paste & the dye molucules coating the hair. The dye molecules will settle soon and the glossing effect of henna will be apparent.
Just wait for a wash / condition & give it a few days.

1 - it could be that the acid you dye released with is too low a pH for you.
If using lemon juice or vinegar, you can dilute them with distilled water (to a maximum of 50% dilution).
Or, try different fruit acids for dye release
Fruit Acid List
2 - you have a fungal based issue on your scalp (dandruff, eczema, psoriasis) & henna's strong anti-fungal properties are working on it.

Henna is not scientifically shown to help with hair loss.

Hair loss is either:
- genetic
- hormonal / illness based
- scalp issue based
- nutritional deficiency / stress
- pharmaceutical drug based
- chemical irritation / allergy reaction based

Which of those could henna potentially help?

Henna is a very strong anti-fungal, so if the cause of hair loss is a fungal-based scalp problem (ringworm, dandruff, eczema), then there is a possibility that long-term, consistent use of henna on the scalp could eventually slow down the excessive have loss.

If the hair loss is from chemical irritation from using detergent shampoo, then switching to only using henna to clean your scalp & ditching the shampoo could help.

I make no claims that henna can help with hair loss in any way... I just want to encourage critical thinking on the topic :)

Henna & Cassia - yes
Indigo - no

Leftover Henna & Cassia paste can be stored airtight in a freezer ziplock bag for up to 6 months.

Leftover indigo needs to be thrown away.

Yes.
If you have brown or dark brown hair, indigo alone can make your hair look black.
If you have greys in your brown or black hair, indigo alone will make your grey hair a varity of shades of blue-purplish blue (but sometimes green-blue)

Indigo alone on light brown, blonde or pure white hair - it will be a surprise! It will turn your hair an unpredictable variety of blue - purple & maybe greens.

But keep in mind: indigo'ed hair cannot be bleached (lifted) later without resulting in crazy green.

the Lawsone dye in henna WILL migrate through the plastic wrap to stain the paper towel. This is the best indicator that your henna paste has started dye releasing.
A faint yellow-orange means the henna is just starting to dye release. Bright orange means it's time to start testing the stain on your hand as per instructions

There can be a variety of reasons why, with the same henna cone of perfectly made henna paste, one person stains really dark & another doesn't (assuming they both followed aftercare instructions)

- skin was just (extremely) exfoliated
- chlorine buildup in the skin
- lotion / oil residue on the skin
- hormone imbalances
- certain pharmaceuticals
- body chemistry (what you eat)
- skin has a low collagen content (age / disease)

Jagua has a *very* high allergen rate. I have read that up to 25% of people will have an allergic reaction it the fruit juice.
No way am I dealing with that!