Omitting arguments in commands

[Franco_64]

I'm a bit confused...

I'm trying to use the TEXTOUT_P command (but it's just an example, I want to find a general rule). TEXTOUT_P accepts several arguments, but I need only some of them.
Example:

TEXTOUT_P (STRING(C), 100, 120, 4, RGB(255,255,255), RGB(0,20,0))

don't give me a syntax error, but when I execute the program I have an "Invalid input" error, I think because the compiler interpret the sixth argument (RGB (0,20,0) as the argument "width", that wants only a number.

But if I use

TEXTOUT_P (STRING(C), 100, 120, 4, RGB(255,255,255),, RGB(0,20,0))

using double commas to "jump" the "width" argument, I have a syntax error.

Ant in this particular situation, all seems to me more complicated because I have omitted the second argument [G] without any error.

What is the correct procedure if I want to use a command but not all their arguments? More in general: where can I find these syntax rules for HP PPL? There is a doc?