If you do decide to go for that cracked, kind of mosaic-y look, I'd probably choose giraffe/hex over pie/paint simply in terms of the former genes selling a little better than the latter. I also feel that ripple/current would be good options. I, personally, don't feel that pie/paint achieve the look you mentioned. I was going to suggest crackle as a tert, but yikes, that gene is a little overbearing. Lace is kind of pretty for a "this is the fancy design on the fine china" kind of look. Runes also has that effect. I like your idea of smoke for the dirt; stained is kind of pretty and could sort of be a "dirty layer"; orca thylacine is interesting and I can kind of see how the shadowing looks a bit like dirt (kinda cool looking paired with ripple/current); orca circuit is faint enough that it looks more like "dirt" than electricity.
And just to help visualize my word spiel, some previews.
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Preferably ones that can be mimicked nicely with scales. So things like robins might not work.
