Nothing wrong with slow and steady, especially if it gives the results you are achieving. Very good start there, well done. You will probably find as you paint more and get more and more used to painting models that you get a bit quicker with each one you paint.
As for your question around the black armour, leaving it black will probably work fine. However if you wanted something for the Jaw piece you could maybe try a white, like in this picture to give you a bit of contrast
Red also works as a good contrast colour against black.
You can highlight the black up using some grey edge highlights to give it a bit of depth. Something like an Eshin Grey for the main edge highlight and then a lighter grey on the highest points, think Dawnstone would probably work (both GW paints btw). Not sure if that helps?