Drupal version 7.0 allowed one not to enter a hexadecimal on the color scheme for themes that support it (Mayo for exemple). With version 7.2 a checker was introduced that would check if the values placed were actually allowed values. Not entering a valid hexadecimal color would yeld the error "must be a valid hexadecimal CSS color value". I scratched my head for a little while and found the color module. If you want to still have transparent backgrounds, just comment out the checker. The only drawback is that you won't get the thing to validade the values for you. In the end I am glad, since using values such as 'black', 'red', 'IndianRed' (for example) are allowable values but the checker only allows for '#000000', '#01adff' and other hexadecimal values.
The fix is to edit the color.module file (/path/to/site/modules/color/color.module). Comment lines 280, 281 and 281 (with // in front of the line). The final result will look like:
//if (!preg_match('/^#([a-f0-9]{3}){1,2}$/iD', $color)) {
// form_set_error('palette][' . $key, t('%name must be a valid hexadecimal CSS color value.', array('%name' => $form['color']['palette'][$key]['#title'])));
//}
you can also comment out the for loop (don't forget to comment out the } as well if you do that).