Painting engine bay?

I am an idiot and should have painted my engine bay while the engine was out but oh well. How can i go about painting it with the engine in the car? It is polo green but is flat so i don't know if i could get anything to match up right. How would flat black or gloss black look if i only did the front radiator support and wheel wells?

Just cover everything up that you don't want to get paint on with newspaper or plastic or something. From my experience the best looking paint for such (inner fenders, core support, etc.) is semi-gloss black. Doesn't shine too much, but isn't dull/dirty looking. Can't go wrong with it.