Why are beetlejuice's fingers red?

Answer 1

When someone dies the blood in their body rushes from the inner body to the outer portions such as your fingers and toes. Since Beetlejuice is a ghost and then is dead, his fingers may be red from all the blood in his fingers.

Either that or he chews his fingernails till they bleed since in the cartoon he constantly chew his fingernails when he's nervous (which is often).

Or he may just paint them red!

You choose.