I actually have a rule about using only one color per post (excluding headers) in my IC threads. My eyes are bad as well.
In general dark text on a light background or light text for a dark background work well. Midtone on Midtone is difficult to see.
If you can't decide if your text color looks good on all the backgrounds you can use the [bg] code to place a background behind your text so it'll look the same on all.
so for example:
Code:
[bg=blue] [color=white]your text here[/color] [/bg]
would look like
[bg=blue]
your text here [/bg]
This is a very helpful tool for choosing colors
HTML Color Groups
If you click on a color name it'll show what that color looks like as a background with the other colors as text on it.[/bg]