If a UIButton's tweenTarget is not set, setting isEnabled on the button doesn't change the button's color unless I've already moused over or pressed the button. Looks like the problem is in UIButton's UpdateColor method:
if (tweenTarget)
{
if (!mInitDone) Init();
// stuff...
}
This version works better:
if (!mInitDone) Init();
if (tweenTarget)
{
// stuff...
}