I vote for grey as the marking looks sort of grey & looks very good on the black. That being said, is this a "practice" piece that is not meant for anything in the end? If so, why not try each of the 4 sections in a different color & see which one you like. I can't imagine you NOT planning on using such an intricate stenciled piece for just practice & that's all but you didn't say what your plans are for it when you have finished. You are really going detailed with your stencil work to get the hang of FM. I wish I was as brave as you. Be sure & let us see then end results.
Can you give us the brand of that marking pencil? I have such a hard time finding something that I like that will give me a fine line like yours.