Pick some colours, then bend how they spread across the picture with two curves: one for left to right, one for top to bottom.
Presets
Your presets
None saved yet.
Colours
Click the bar to add a colour. Drag the markers to move them.
Make a palette
Curves
Click to add a point, drag to move it, double-click to remove it. Across is the place in the picture, up is how far along the colours it lands. Show curves, above the picture, puts the same curve on the picture itself.
Blend
How the top-to-bottom colours are laid over the left-to-right ones.
Canvas
Save
Switch on what should move. Everything is back where it started when the loop ends, so a saved video repeats without a jump.
Save video or GIF
A GIF only holds 256 colours, so it looks rougher than the video and the file is much bigger. A smaller size, fewer frames and no flickering grain all keep it down.
What gets saved is one pass of the loop. Save picture, above the preview, saves the moment that is showing.