I'll tell you a funny story about blue solder mask.
The company I work for always had blue solder mask as an option, but very few customers actually ordered it. But one customer did, basically to differentiate versions of his product.
Anyway, he ordered one lot, and was quite happy with them.
Then a month or so later, he ordered some more.
In the meantime, we had started using a different brand of FR4. The new FR4 was better for us, because the fibreglass was coloured yellow to filter out UV light. (UV light is used for imaging the panels, and the idea of filtering is to prevent the light from one side of the board from passing through the fibreglass and imaging the other side).
This guy's boards weren't too heavily covered with copper, so the blue solder mask was applied to mostly bare yellow FR4.
What happens when you mix yellow and blue?
Yes, the boards came out green They looked almost exactly like the green solder mask we normally used.
He was local, so he came to visit us, with a sample.
He said, "I ordered blue boards!"
We said, "But they ARE blue. We used blue ink."
He said, "But they're NOT blue.. They're GREEN"
We said... "No, really.. they're blue!"
He did accept them in the end. And we started buying different solder mask ink that works with yellow FR4.