Has bugged me for a while, however never spent time on looking into it... is this a bug or am I missing something?

When selecting one of the stock Images, i.e. arrows that have a blue infill, when I try to assign another colour to them it is as if the colours "merge" to make the resultant colour see below example, blue arrow, configured to yellow, shows green..

It's because this is a colorize tool. The blue gets a yellow overlay creating green. Most of the graphics which are colorized are grey by default so the colorize outputs colors matching the selected hue.