cmap = winter(10));
cmap(1,:) = [.7 0 0];
Ztemp = max(0, Z);
pcolor(X, Y, Ztemp);
When you use pcolor(), the Z coordinates are all set to 0, not to the value of the data, so it does not matter that your Z is not the "correct" Z, you won't be able to tell with datatips (not unless you use a custom datatip function.)
There are alternatives to pcolor() that do not set the Z to all 0, and for those alternative functions, it can be a bit tricky to arrange that all negative values are one color without affecting the correct functioning of the Z coordinate in datatips.