I am writing this thread because I want no one to go through what I experienced.
I had long (past breast) hair that was lightened with Loreal box dye to a strawberry blonde.
I decided that I wanted to go darker to keep my hair in good shape and grow it even longer.
So after some research I found Katam. Online reviews (there wasn't very much info about it) said it didn't have the blue tint that indigo has but rather a purplish tint.
So my thought was that in the worst case scenario I could totally rock purple hair .

I of course did a test strand of first applying Henna and then an application of a Katam and Henna mix. The test strand turned out like I wanted (auburn brown).
So I used the same procedure on my whole head of hair.

To my great dismay and utter shock after washing out the mixture my hair was auburn brown with emerald GREEN streaks!
Parts of my hair were the shade of the test strand while the rest was reminiscent of Anne's of Green Gable's hair dying disaster.
Whichever strands of my hair were processed most by chemical hair lightener in the past reacted terribly with the Katam EVEN THOUGH I applied it on top of Henna.

After this nightmare I tried everything to get rid of the green but NOTHING, absolutely nothing worked.
Vitamin C: nope. Color OOps: nope. Sun in: nope. Bleach: nope.

I had no choice but to cut out streak by streak of that horrible swamp color.

This happened 8 months ago and I am still recovering. There are still some light greenish streaks in my hair and of course I lost a lot of length to get rid of the dark emerald green.
It looks okay-ish now (I can leave the house lol).
Most people think the streaks are a very ashy dark blonde (which is not ideal with the rest of the hair being reddish dark blonde) but I can manage.

It will take me a few years to get back to having "green-free" long hair. Sigh.

Please be careful with Katam it is extremely unpredictable especially on chemically treated hair or light hair.