I can confirm that the colored bar under the new slots indicate what you can put there. I also tried to put something yellow (universal) there but it didn't work. Only blue if blue is selected, only red if red is selcted etc.
Overall I really like it, cudos to CDPR for taking this suggestion from the community for more slots which many people wanted! I haven't played much with it yet but looking at the mutagen menu it seems you can make the signs have 50% of your sword damage if you go down the blue route, so that might make signs more viable towards end game which I would like very much.
Overall it seems like a great new system to try out and play with, lots of combinations and options.