On the forums we can select the color by using the bbcode tag COLOR, like so (replace the {} with []):
However, the tool when editing a post allows for RGB too, but the forum doesn't support that:
Code:
{color="rgb(255, 255, 255)"}
Here you see both methods applied to text:
This text should be red
[color="rgb(255, 0, 0)"]This text should also be red[/color]
The way this was discovered was by clicking the color button once, selecting a color, then clicking it again.
Keep in mind that some colors will use strings (like red), but odd colors like Plum will write the rgb version.