Each gear piece has a stat cap for each stat. If it's red, then the gear has reached this cap, or will exceed the cap when the materia is applied, so try using materia of another stat.

If all of the gear is capped on those stats then the only other thing you can do is see if you can get a higher item level of gear.

For battle jobs you can go to a materia melder NPC and meld it so having a crafter isn't required for those.