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I am looking for a way to replace a stroke done by the Sketch Panel pencil tool with a custom color similar to the ink bottle's function in the Sketch Panel.
Is this possible?
To be a bit more detailed: I'm working on a project now where I'll have one layer per color in my animation, say I have one layer with the animation's brown color, one with yellow and so on. I'm using the normal Sketch Panel pencil to block out these areas on each layer.
But I don't know what to do if I later decide to change, say, the brown layer into purple.
I'd be grateful for any tips.

One of the colour correction tools of the FX stack should do the job.

Olve wrote: But I don't know what to do if I later decide to change, say, the brown layer into purple.
I'd be grateful for any tips.

fastest is to switch on the alpha lock (last menu button at the top next to the proxy bar)
and then color the whole surface of your project with either the bucket (range set to 255) or the rectangle fill tool.
when you need to do the whole animation, select all frames(double click under the layer) and press enter.

There is also the FX Color > Color Replacer.

In the exemple below, it converts the red into yellow/orange
( panel in french sorry )

I had a look at the FX Stack, but didn't really figure out how to get the color I wanted (I only found a slider giving me seemingly random colors). Then again I didn't spend too much time getting into it.
Anyway. Your suggestion Peter was exactly what I was looking for. So thanks a lot.