Hrm. I didn't know you could rgb(255,0,0) but I looked it up at
http://www.w3schools.com/css/css_colorsfull.asp and there it is! Cool.
Anyway, I believe that colors not supported by the browser are dithered no matter if that color is like #F80000 (as in CSS background colors), or if the color is in an image.
So I really don't think tiling an image of a certain color will help you (if the browser doesn't like that color..it doesn't like it).
I would recommend just setting your background color using CSS, etc. to exact color you want (like #F80000, or rgb(248, 0, 0) ) so that if the browser likes it..it displays, otherwise it uses a dithered color.
Hope this helps