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Well, it would only be a light tint layer (with some acrylic blender to protect and smooth the graphite), not a full color conversion. I'd just try to to give it a vintage render.
The idea behind these small drawings being that they could be old souvenirs from a travel in an imaginary universe.

Sounds wonderful. Ensure you don't loose those wonderfully moody expressive marks. Love it

Thank you, I'll try :)