You can now have any color gui you want in bedrock edition with a resource pack!
I have been working on a tool that allows people to create resource packs in java and bedrock edition to customize certain things in game on a deeper level and finally got the bedrock version working for guis!
For this version you will need to download a resource pack made by CrisXolt (dont worry I made sure to ask for permission!) because I was not familiar with bedrock code.
I would appreciate feedback! See comments if you wanna check it out!