Miguel -
Two choices:
1. Put some JavaScript/jQuery to check the CSS based on the SELECTED attribute of the checkbox.
2. Put a "style" Extended Property on the checkbox with a value something like: if(Form.Record.Entity.Attribute, "color: blue;", "color: liliac;") or whatever CSS works, then add an "OnChange" chander to the checkbox that calls AjaxRefresh on the checkbox.
J.Ja