At present the disabled state of the labels is carried out by the browser - in IE the label text becomes "embossed" and semi-transparent / grayed-out in Firefox and Opera.
Currently, there is no CSS workaround for this in Internet Explorer, because no CSS can be applied to items that have the disabled="disabled" property.
However, in Firefox and Mozilla you are allowed to manipulate disabled items with css. Here's how this could be done with the Default skin of RadFormDecorator:
color: red !important;
Do not add space between classes, as this is a composite class (i.e. the three classes are applied to the same element) and do not forget the !important flag, otherwise you will not be able to override the default settings. If you are using a different skin, just change Default with the name of the skin you are using. You may also play with the transparency values - opacity and -moz-opacity. Any property value you are using for this fix should be followed by the !important flag.
the Telerik team