Add alternating colour mode

[ace-dent]

Enhancement:
Add a new 'colour' option - alternating mode.
Drawing will invert the colour of the existing pixel (black --> white / white --> black).
If the pixel is set to transparent, set to white on the first draw.

This is very useful in the pencil drawing mode. It saves time avoiding manual colour changes.
When the background has been set (flood fill), we can just draw the foreground details. If a mistake is made, a second click will restore to the background colour.
Various other pixel drawing programs act in this way.

Re. icons - I'd suggest using the current 'invert' icon - as the square form will fit well under the other colour boxes.
A new icon for the global invert tool, could incorporate a round element (which is common). e.g. https://www.flaticon.com/free-icon/invert-colors-button_61136

[ace-dent]

My first suggestion was essentially: 1st click 'on', 2nd click 'off'. This duplicates the behaviour of bitsy and other editors. I'm not too sure how well that fits with the overall design of your program?...

Alternative: Keep single colour draw on left click unchanged, just add right click for colour sampler/ selection. This will act to speed up drawing, as we can change colours without having to move the mousing from the editor to the palette. This approach is used in PICO-8.