I would suggest moving the common styles together:

QLabel#r_label,
QLabel#g_label,
QLabel#b_label,
QLabel#a_label
{
    font-family: FIXED_WIDTH_FONT_FAMILY;
    font-size: 11px;
}

Then you can just set the color of each label:

QLabel#r_label
{
    color: red;
}

Etc.