I don't mind the use of colour, so long as people aren't using it as a crutch. For example, I tend to get a lot annoyed when I see people using colours to differentiate between character dialogue and think that's an excuse not to format properly (beginning a new paragraph when a new person is speaking, using paragraphs to separate topics, etc.)

As well, and this is more of a personal problem, I'd rather people use colours that are easy to see, but not too bright. I have very photosensitive eyes, so I highlight all the text here in order to read it. Therefore, when someone uses say, bright red, it's hard to read both highlighted and not. 

Basically, it's not a bad thing. I don't tend to use it myself unless I feel like readers are going to skim over my post if they don't see colour. I do HATE it, though, when people use colour as an excuse to ignore proper grammar and formatting. /end rant