Add custom functions to be parsed?

Not really an issue but not sure either where to ask that question.

I am wondering about the possibility to ass custom functions to be parsed. My use case is very simple :
`rgb(123,123,123)` will be parsed and a color will be displayed.
In my case I'm using an R function which is very similar : `rgb(123, 123, 123, maxColorValue = 255)` where the fourth argument will always be like this. Because there is a fourth argument, it is not currently parsed as a color.

Any way of doing this kind of things ?
Apologies if this is not the right place to ask that.
Thanks ahead of time for the help :)