By default `l3draw` only supports `red`, `yellow`, `blue`, `green`, `white`, `black`, `cyan` and `magenta` (and combinations thereof). 

How to define more colours?
Top Answer
Under the hood `l3draw` uses `l3color` for the colour support, therefore on can use the `\color_set:nnn` or `\color_set:nn` macro to define new colours.

One example with an rgb colour (the colour models `gray`, `cmyk` and `spot` are also available) and based on existing colours.





\color_set:nnn {foo} {rgb} {0.5,0.8,1}
\color_set:nn {bar} {yellow!50!red}


	\draw_path_circle:nn {0cm,0cm} {1cm}
	\draw_color_fill:n { foo }
	\draw_path_use_clear:n { fill, stroke }
	\draw_path_circle:nn {1cm,0cm} {1cm}
	\draw_color_fill:n { bar }
	\draw_path_use_clear:n { fill, stroke }	

How to define custom colours in l3draw
samcarter replying to Skillmon
Skillmon replying to samcarter
much better!
samcarter replying to Skillmon
samcarter replying to Skillmon
mmm, maybe I could simply make two examples. Let me try...
Skillmon replying to samcarter
then you could say by the "family" or something like that. `\color_set:nn` is like `\colorlet` and `\color_set:nnn` is like `\definecolor`, so they are quite similar in their result but different in their inner workings.
samcarter replying to Skillmon
But how to decide how many n's to use? The macro exists both for `nn` and `nnn` (I hoped I could duck this decision by only taking the front part )
Imho, in text you should call l3-macros by their base-name which includes the arguments `\color_set:nnn` instead of `\color_set`.