Blue Cherries on a dyed blue PCBI dyed some PCBs blue. The soldermask was sanded down to the bare FR4 fiberglass surface. I tried 3 different dyes, each for 15 minutes simmering on a hot plate. The first dye was Rit Denim Blue. After 15 minutes there was no noticeable coloring. The second was Rit Dyemore Blue. Also no noticeable change after 15 minutes. Finally iDye Poly Blue. After 15 minutes it was a deep blue. I left it in for another 5 minutes and it didn't seem to get any darker.
I found this post where it took almost 2 hours to dye with regular Rit dye. So it can be done if you have a lot of time.
I have also dyed some purple PCBs.
The keycaps are from a PMK grab bag. They are upside down.
Six Pack with Outemu Teal switches.
Gherkin plate and a 2 pack plate at the same time.