Itching settling down, but now flaky scalp

I did my first henna (hendigo) about 3 weeks ago.  Because my greys are so resistant I  shampooed with dish soap to help prepare them for color.  I used grapefruit juice to mix the henna.   My scalp "tingled" (more like a stinging) while the henna was applied, and I actually developed a few small scabs over the next couple of days.  Plus my scalp has itched something awful up till a couple days ago when I deep conditioned my hair.  So now the itching isn't as bad but my scalp seems to be flaking more than before, a fine white flake when I scratch.

Was the irritated scalp probably from the liquid dish soap shampooing, or a too acidic henna mix, a combination of the two--or maybe something else?  I read somewhere on the previous forum that using these fruit juices can be equivalent to doing a chemical fruit peel on the scalp that takes a while to "heal".  But I do want to get the most lasting color (I'm doing a two-step process this time) which requires a more acid liquid, right?  What can I do to protect my poor scalp and hair, while getting the longest-lasting color?

Hard to know exactly what it was, but I can imagine the combination of all the above could be a bit much for a sensitive scalp.

I'd try changing one thing at a time, so you can find out what makes the difference.

Since citrus sensitivity isn't exactly rare, you could try using something adequately acidic but non-citrus like apple juice next time.

If you haven't already tried and failed with a simple clarifying shampoo instead of dish soap, you could give that a go.

Many people do deep conditioning right after they henna, or at the very least they rinse out the paste with conditioner. Oiling your scalp overnight might be soothing. If you're concerned about avoiding anything that could impact the indigo, wait 24 hours before you do the DT or the oil (these things may or may not have an effect on permanency with indigo -personally I've never seen it, but I haven't dealt with resistant gray). Using plentry of conditioner to rinse should be okay, though.