Why is the background going blue?

Cap

If I use background(mouseX), it goes from black to white as I move the mouse across x but at 255 it goes from black to bright blue instead?

GoToLoop

Up till 255, the argument value is considered as grayscale.
After that, it is interpreted as an aRGB color. L-)

GoToLoop

You can use map() in order to confine mouseX's width range in a grayscale range: *-:)
background(round(map(mouseX, 0, width, 0, 255)));

https://Processing.org/reference/map_.html

GoToLoop

Alternatively to map(), you can go this way too: :ar!
background(mouseX * 255/width);