REQUIRES OPTIFINE
Warning: flashing
This resource pack for Minecraft: Java Edition makes glow squid alternate between colors of the rainbow.
A video showing it off: https://youtu.be/EY88a4bTbSA
(It's much smoother than it looks)
TO CHANGE THE SPEED AT WHICH THE COLORS CHANGE:
Extract RainbowGlowsquid.zip
RainbowGlowsquid > assets > minecraft > optifine > anim > squid > glowsquid-rainbow.properties
Open the properties file in Notepad or any other text editor
In each repeating module, the number to the right of the equal sign after the word "duration" represents the number of game ticks in between each color.
Adjust that number for every module (It's automatically set to 12, higher numbers mean it alternates slower)