rgb() is deprecated, it's left for compatibility with old project.
There are four expressions you can use - rgba, rgbex, rgba255, rgbex255
The first two take values from 0 to 100. For example rgba(100,0,0,0) is red.
The other two take values 0-255, rgbex255(255,0,0) is red.
You can find all this info in the documentation:
construct.net/en/make-games/manuals/construct-3