What about a transparent png icon with a background color applied via CSS?
Images would all be the same size but you could alter colors based on transparent areas of the PNG.
This is a jpg representing the idea....

But the actual png would have the actual icon transparent:

<img src="smile.png" width="100" height="100" border="0" class="evaluser0" />
<img src="smile.png" width="100" height="100" border="0" class="evaluser1" />
<img src="smile.png" width="100" height="100" border="0" class="evaluser2" />
<img src="smile.png" width="100" height="100" border="0" class="evaluser3" />
img.evaluser0 { background: #fef200; }
img.evaluser1 { background: #f26422; }
img.evaluser2 { background: #ec145b; }
img.evaluser3 { background: #00a650; }
red > red-orange > dark orange > light orange > orange-yellow > yellow > yellow-green > lime> green– poepje Jun 22 '12 at 9:29