Illustration not working with transparency

So I drew a simple illustration in PS using the brush tool on a separate layer, I was working on a white background, and it turns out I need this illustration to work on different colours of backgrounds, so I need it to be transparent, with just black lines on a transparent layer. The problem is, I used some gray lines in the drawing too so it doesn't work on different backgrounds than white, it just looks super weird. Is there a way to fix this?