I really liked the theme and very much admire the construction. I thought it was very clever how instruction D worked in harmony with the theme, and my only issue at first was choosing poor colours that made me consider the non-C features in an inverted way! Also happy enough with the letter change, based on the final hint in the preamble.
(I do wonder if anybody fills in their grid in a slightly different style which will not have worked for this endgame!)
However the colouring has caused me some consternation. I know, it's just for fun, but this crossword has stats and one is recognised for an all-correct year. That changes the game for me; I wish it didn't, but that's my brain.
The preamble is unambiguous: using D, "solvers must
show a picture", and "C’s corrections spell what must
then be added by changing
one grid letter". "Then" clearly indicates the picture is to be shown first and the change is to be made afterwards. The nature of the change is one letter; if it were more than that, it would have surely said so. But this leaves me unsatisfied as the final picture (squinting my eyes and look at the colours only) represents the
problem, not the
solution.
I will be submitting the result of following the instructions precisely, and not making the additional change that would show the solution fully. If we're to guess at the expected correct submission, this is surely the one that's indicated by the text. It's not satisfying though, and it's a shame that the ambiguity is there. (Even if either possibility is marked correct, as has happened before in similar situations, it's too late in the sense that we're already fretting over what to submit.)
To not end on a low note: I really did think this was very clever.