Ahh, a painting question! There are several types of plastic on the X so, it makes a difference. I painted my plastic grill and wiper motor cover using just Krylon blue paint. First clean it well with wax/grease remover (you can get that at any auto parts store) and then some acetone. Scuff all the parts with fine sandpaper or a fine 3M pad. I prefer the 3M pad because it get around corners better. Apply several thin coats of paint, let it dry, and you are done. I really like Krylon for rattle cans. It doesn't run easily and overspray melts right into the paint. Rustoleum sucks.
Now for the real way to paint plastic, particularly softer plastic like the mud guards, it is more involved. Clean the plastic as before and warm in a oven for a half hour at 140 degrees, being careful not to melt the plastic. Clean again with acetone and allow to cool. Scuff the plastic as before with a 3M pad or sandpaper and spray on plastic adheasion primer. This is available at automotive paint stores, but you'll need a spray gun and compressor as it doesn't come in cans. After it drys to spec, you can spray on the paint. I have only done this with automotive color/clear coat paints. It works really well, but is a fair amount of work.