Why is it that in the show, Ickis always looks red, but outside of the show, he's purple?
Because the show was made in the 90's when characters and backgrounds were painted by hand. Later merchandise is colored by computers which cannot reproduce all the shades that were possible by mixing paints.
I'm not sure I buy that. I mean, I buy that digital coloring can't create as many subtle shades of color as hand-painting. But purple is a whole different color, not just a different shade of red.
Ickis describes himself as being magenta in "Attack Of The Blobs". Just looking at different brands of art supplies, magenta can be depicted as anything from a pinkish red to a purple shade. Within the show itself, other characters have described him as red (Bradley in "Into The Woods"), or purple (Zimbo in "Side by Side") so it could also be a matter of Hair Color Dissonance.
Is it ever explained why the monsters must scare people? What do they get out of this?
It's mentioned several times that monsters are the incarnation of human fear of the unknown, and only exist as long as this fear exists; including basing one entire episode around monsters disappearing because people no longer fear them.