I use both Windows and GNU/Linux and I know that mIRC will not ever be a GNU/Linux application, but when I am in GNU/Linux, I would like my experience with X-Chat to be as similar to mIRC as possible, and was therefor wondering if anyone could post mIRC's default html codes for all 15 colors that mIRC uses. Then I could change my colors on X-Chat to match so that text looks the same no matter which client I am using.

open mIRC > view > colors > right click a color > click 'define custom colors >>' > write down the RGB value > convert the RGB value to HEX for HTML colours

Why dont you use mirc on linux?
saves having to work out the colour codes