What color block? What kinds of paint?
What color is your engine bay going to be/ already is?
Then find a nice color that contrasts that.
My bay is going to be a medium metallic blue, so I will paint my engine orange.
If it is a new rebuild or an old engine, I suggest washing and scrubbing the engine with strong soapy water until there is
zero trace of grease or oil and rinsing it down w water, followed by "metal prep wash", let dry completely, then epoxy primer, then your color.
Eastwood makes a variety of engine colors and also a metal prep wash:
http://www.eastwood.com/paints/hi-temp-engine-paints/engine-paints.html
http://www.eastwood.com/eastwood-afterblast.html