Concealment causes color highlighting to not apply

Question

Following my previous question, I have the same string aaabbbb that I am colorizing in the same way, except with the addition that I am now also trying to conceal the as.

Here is the best that I got so far:

" here xB is not matched
syntax match xA /^a\+/ conceal transparent
syntax match xB /\(^a\+\)\@<=b\+/
highlight xA ctermfg=red
highlight xB ctermfg=blue

But the problem is that, while the as get concealed as expected, they don't get colorized.

But, if I remove the concealment, colorization works fine. I.e. this colorizes just fine (but obviously doesn't conceal the as):

" here xB is not matched
syntax match xA /^a\+/
syntax match xB /\(^a\+\)\@<=b\+/
highlight xA ctermfg=red
highlight xB ctermfg=blue

Q1: How to colorize and conceal at the same time?

Q2: I feel that I am missing some fundamental knowledge about how vim applies match/highlight. What is going on here? Is the concealment causing vim to not see the text during the syntax highlighting stage? I mean, it's the same match statement. So vim saw the text and applied the concealment, but somehow thought to not apply the color to it. What's am I missing in the internal vim workflow?

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Update: This seems to work, but I am not sure why.

" here xB is not matched
syntax match xA /^a\+/
syntax match xB /\(^a\+\)\@<=b\+/
highlight xA ctermfg=red
highlight xB ctermfg=blue
syntax match xAConceal /^a\+/ conceal transparent

Answer 1

IIRC, transparent prevents coloring the text in any ways.

I would expect the following to work

syntax match xA /^a\+/ conceal
syntax match xB /\(^a\+\)\@<=b\+/
highlight xA ctermfg=red
highlight xB ctermfg=blue