I thought this might be like the ZX spectrum, but that covers each 8x8 block with a foreground and background colour; "sprites" are then bitmaps over that. This does say two colours per pixel, which is confusing.

Here's the docs for the screen device: https://benbridle.com/projects/bedrock/user-manual/screen-de...

You set up a palette of 16 colours, then write 0-15 to the coordinates where you want to set a pixel, but you can also choose between an overlapping foreground and background layer (colour 0 on the foreground layer is transparent).

I guess it's no more weird than some hardware designs from the 80's...

I made a fantasy console with 3x3 pixel cells defined in 16 bits to do any two of 16 colours in the cell

    4 bits : color A
    4 bits : color B
    8 bits : select A or B for the first 8 pixels of the cell

The last Pixel is always color A. You can independently change all pixels in the cell because changing the last pixel on its own can be done by swapping A and B and inverting the second byte.

In hindsight I don't think there was much advantage to the last bit being the odd one out. The code for setting individual pixels in a cell was pretty custom anyway. If I were to do it again, I'd place the color A pixel in the center.

And I do find myself working on a memory constrained device again, so perhaps I'll be giving it a go.

Each screen pixel has two colours because there are two screen layers, a foreground layer and a background layer. Anything you draw to the foreground layer will be draw over anything on the background layer, so you can use the foreground layer for fast-moving elements like mouse cursors and game characters without having to redraw the entire background every frame.

So each pixel has a colour on the foreground layer and a colour on the background layer, and will be drawn as one or the other. Normally the foreground colour of the pixel will be the colour used, but if the foreground colour is palette colour 0 (treated as transparent), the background colour will be used instead.