Hi all. Thanks to Yann I'm finally also doing plugins and have managed to realise something that I have felt the need for for a long time...
It's super-basic. What it does is take the human-friendly values of Hue, Saturation and Brightness and turn them into computer-friendly values or Red, Green and Blue.
And then you do things like this:
Easy as pie:
It has no actions, no parameters for edit time - all it does is convert your HSB values into the RGB type that Construct Classic likes. So you can use it wherever that is welcome, like, say, right clicking on sprite filter colour dialogue and picking "Use Expression".
Here is the plugin. And here is a quick example.