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Industry bike color chooser and designer of paint schemes here.
A lot of it comes down to cost, and ease of replication across many frames.
If you're a custom bike builder or doing small frame runs, you can experiment a bit more. Firefly does some interesting things with anodized titanium.
Fades are easy. Solid is easy. Decals are really easy (once you account for the different frame sizes). All of that can be done by fairly unskilled labor. Anything involving harsh chemicals, acid, masking or tightly controlled environments are usually out question.
Pic related, this had to all be masked off, and then have varying controlled voltage run through an applied acid solution as they move up the mask. If they mess up you have to sand it all down and start again, which isn't great since this is usually done at the very end.
