What ever you do, put up a picture! Brown bats, gold bats..if you look in nature's true colors there are all shades! So go for it!
I used a twisted black/white thread on black adn it looked like dark grey :)
If the shirt is a mat black Stitch in a glossy black or stitch the outline on a straight sewer or by hand with a thread that shines in the dark. goodluck
If there is an outline could that color be changed?
Yes, I would use a darkish grey. I did a tiger on a black jacket with a dark grey, it came out relly good.