An excessive palette of colours distracts, as per this answer. However, what of adding simply one more colour?
1 key reason: Suppose you're quoting a long text that already uses bolds and italics. Then all formatting tools have been exhausted; no tools remain. So how can you highlight something?
The following posts already use or substantiate the utility of one more colour:
- Is there a way to change the color of text in a question?
Adding feature to colour the text, was broached for French SE.
I cite a supportive comment from moderator 'snailboat' below:
For Japanese, where italics aren't often used and look strange, it would give us something other than bold to use. (The Powers That Be have already forbidden underlining, which would also be very useful on language sites.)