What would cause this simple paint to act so weird??

[johnstonf]

See video...

Just trying to paint on a layer!

Have rebooted my MACbook Air, still the same.

https://youtu.be/DVLkInrLJmU

[Andrius]

At the bottom of the Brushes palette, there are three sliders in the Color Jitter category: Hue, Saturation, and Lightness. When you move the Hue slider, the hue of each brush mark is randomized. The more you move it to the right, the more random (i.e. more different from your selected color) the colors are. When you move the Saturation Jitter slider to the right, the color intensity of each brush mark is randomized, and when you move the Lightness slider, the brightness of each brush mark is randomized. If you move all those sliders down to 0% (or choose a different, non-jittered brush), the brush will go back to 'normal'. 😉