`strtol()` requires including `stdlib.h`, but you could also use one of the GDK functions such as [`gdk_rgba_parse()`](https://developer.gnome.org/gdk3/stable/gdk3-RGBA-Colors.html#gdk-rgba-parse) to parse the colour string or expose [one of the functions](https://github.com/geany/geany/blob/5cc69b3d6728a714637caabf0238759363e91e85...) already in Geany to the plugin API and use that.