It's a tough call with all the cast finish aluminum and magnesium.

Black won't show up very well IMO.

Purple might be okay with the amount of bare looking cast finishes.

Silver may be another option, but usually I trim the engine in black then (valve covers, etc.)
The color silver "pops" (stand out) when you put in in a purple engine compartment.

I know, not a Mopar engine but it's a silver engine in a purple car.

Street Hemi orange would probably be my first choice though.

To me the engine needs to contrast to the body color to make it stand out some, like a tunnel ram Hemi isn't going to stand out.