My newest resource pack for 1.8.9. you can have another pack underneath it so it becomes an overlay. All it is, is a rainbow hotbar and xpbar with the colors red, orange, yellow, green, cyan, blue, purple, and pink. I also changed the crosshair a bit, but its not too different. here is the 1.17 version --->
www.planetminecraft.com/texture-pack/rainbow-gui/