1) RGB: #rrggbb (rgb hex) / rgb(r,g,b) (rgb int 0-255) / rgba(r,g,b,a) (as before, plus alpha). In addition, rgb/rgba support percentage values instead of 0-255.

2) red (color names)

3) hsl(h,s,l)

4) cmyk(c,m,y,k)

For compatibility with anything except Thunderbird (which supports all four formats IIRC), stick with #rrggbb or plaintext names. Webmailers are especially prone to filtering by regex (ugh), and stripping out everything they don't know.