Paint

These are the paints is 'mainly' use.  They are water based and
even water-proof.  They can be scratched off if you try hard
enough.  I like them for two primary reasons:

1.) They mix well and w/ a decent # of different colors, it's
relatively easy to mix the exact color you need.  Colors w/
flourescents are impossible to mix w/ these but most of the
plastics that Hasbro uses are 'do-able.'
2.)  When the paint is fresh it dries very smooth, unlike many
oil based paints which can dry un-even and sometimes lumpy.

Below on the left is the type of oil based paint that I've found to
work best.  I use it in certain situations, like when painting a part
that gets an unusually high amount of friction which will chip the
water-based paints above.

Below on the right is a 'varnish' I've used on top of the water-based
paints shown above.  It dries clear and smooth and adds a good
extra layer of chip/scratch protection.