Allow choosing any fill color

[martinvanzijl]

Fixes #63.

Demo

Here is a video demo:

Before merging the PR

• [x] Add unit tests that cover any added or changed code.
• [ ] Update the help file.

[martinvanzijl]

@cvz What do you think of the demo above?

[cvz04]

Demo looks perfect. The color palette is big enough to allow the user a reasonable number of choices.