I love the paint mods. I also love shield mods/titanium. I hate that I can't have both!
Why are paint mods and material mods exclusive?
Paint mods aren't all that powerful IMO since it only works if every part of the armor is painted that same way and if one or the pieces gets broken you no longer get the bonus. Also the bonus is pretty small, only a +1 to whichever special.
Material mods on the other hand are item specific and tend to be much more useful. I like to put titanium on the extremities since they tend to break more but have explosive/energy shielding on the helmet and torso.