Traditional novels have never felt the need to literally colour their dialogue and I kinda buy the argument that, if you have faith in the quality of your writing (certainly writing that is comparable to traditional novel conventions), you shouldn't need to visually differentiate between one section of dialogue and another; the words should do that for you, and colouring your words so that they don't have to seems like it's asking to get into bad habits if one ever turns their hand to a more 'serious' all-text project. If one considers plain text to be 'tedious' or 'monotone' to the point they're distracted then they're either not reading it or it's bad writing. Either way, using colours to fix the problem is only addressing the symptom of (in)comprehensibility and not the root problem of the writer or the reader not being fit for task.

Then again, one of the things I love about the internet is how it facilitates the challenging of and experimentation with the form. Who says, now that we have options, all writing should be in the plain text format? Including music and pictures and maps and even videos is completely viable on forum pbp, and, [i]if it's thought-through[/i], any and all of these can be useful tools. Does colour have a role in this? Probably. I see people using hiders and indentations and horizontal lines and colours to divide up whole sections of text with different tones; perhaps a brief glance into another POV or a flashback. Somebody used colours extremely well in the game I'm GMing to simulate a text message:

[quote=@AuntFlavia][center]
[sub][color=7ea7d8]everything okay J?[/color][/sub]
[sup][color=00746b]Yeah I'm fine. Thanks for breakfast. <3[/color][/sup][/center][/quote]

I'm reminded of one of my favourite features of the BBC series Sherlock; having text messages and electronic content appearing on-screen for the viewer. Playing with the form can yield new ways of audience interaction, and, whether it's superficially slick or integral to the themes/content of the work, experimentation with art is, you know, generally a good thing in my book.

That said, being thought-through is key. Writing dialogue in colours, and, particularly, when each character has a consistent and unique colour in a game, if Mr Pink and Mrs Orange ever get in a long back-and-forth exchange, it's going to be utterly revolting to even look at on the page - which is not going to help distinguish dialogue for everybody, because it may simply become distracting for some. And, obviously, the level of experimentation players should feel free to indulge in should be for the GM to decide.