Is there any way to create transparent color selections in RW’s color picker?
You could implement this : https://bgrins.github.io/spectrum/#modes-input
I recommend it.
Depends on what you are doing. If you are using Stacks 3 and if the Stack Developer has added the option, you can set transparent colors in those stacks.
Can in RW 6 to do the same panel, with a return to colour by default (as in RW 5)?