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.

```
\documentclass{standalone}

\usepackage{l3draw}

\begin{document}

\ExplSyntaxOn

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

\draw_begin: 

	\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 }	
	
\draw_end:
\ExplSyntaxOff 

\end{document}
```