I made this on ibisPaint

Ok so Open ibisPaint then Fill the canvas with a color of your choice, then airbrush other colors onto there (trapezoid airbrush idc which one) Then Make a little dot of Black Or White in an area where you're not gonna draw in (preferably a corner) then go to special pen and click expand and make sure the size is the max, then just hover and hold on the dot until it covers the whole canvas. Finally, change your special brush to revert, change the pen size, and just draw (don't add layers)