Currently StackOverflow (and maybe all other portals) cannot clearly differ between "good" and "bad" code. So, when quoting a users code it's not really visible what's wrong and what's not. Giving users the possibility to color text red would make some thing much more clear.
1. LET'S MAKE IT RED
Don't you hate it when users think that CAPITALIZED BOLD QUESTIONS WILL GET MORE ATTENTION?? Well... I do. And I don't want it to be possible for users to color the whole post or even some parts of the post with an annoying color such as red.
2. This is not an IDE
All IDEs and coding tools use decent colors to clarify critical errors.
Stack Exchange is not an IDE. It simply offers formatting tools to make the code clearer. Trust me; it's already clear enough. Also, when I see colored code, I don't see mistakes or wrong things. I see classes, methods, data types. Mixing errors with the whole thing would be just messed up.
3. Aim for the right solution
Like stated in a comment above, text will always be the best solution here.
This is good:
int x = 2 / 2;
This is wrong:
int x = 2 / 0;
Why ? It's more clear (you don't have to think about "hey what is this color's meaning again?") and also, we are not forgetting our color blind friends.
I see too many problems here for this to even be considered.