Since 1.20.5's command changes, the particle command now no longer is able to display a range of colors at once with /particle entity_effect. 

This is really upsetting because I can no longer have the rainbow effect that I wanted in my minecraft map. (see the before and after images, now I can only make it display one color at a time.)

I think a new component for entity_effect would fix it. Possibly something like this:

/particle minecraft:entity_effect{color:[0.8, 0.8, 0.8, 1.0],delta_color:[0.2, 0.2, 0.2, 0.0]} ~ ~ ~ 5 5 5 1 100 force

[delta_color being how much the color values can be offset by (each individual color value (r, g, b) could be 0.6 to 1.0 in this example, making multicolor a possibility)]

This fix would restore the lost functionality of entity_effect, maybe even give it more functionality than before 1.20.5.

"Before" screenshot's command: /particle minecraft:entity_effect ~ ~ ~ 5 5 5 1 100 force

"After" screenshot's command: /particle minecraft:entity_effect{color:[1.0, 1.0, 1.0, 1.0]} ~ ~ ~ 5 5 5 1 100 force

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I also tested this command: /particle dust{color:[1.0, 1.0, 1.0],scale:1} ~ ~ ~ 1 1 1 1 100 force; there still seems to be some color variance here. (More color variance when you increase the color values [ex. 3.0, 3.0, 3.0])

Perhaps delta_color could be added to this particle too, and falling_dust, and dust_color_transition (delta_from_color, delta_to_color)?

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Another thing I would like to mention for future reference: Minecraft really needs better version control for their commands and command blocks. (Something like backwards compatibility, or conversion of commands in existing command blocks.) 1.20.5 broke a whole bunch of stuff in three of my command block worlds due to lack of proper version control, and it is going to take me a while to fix. (item component system, particle command, change of case for certain NBT tags such as Item:{Count:1} to Item:{count:1}).

The same thing happened a while ago when upgrading worlds from 1.12.2 to 1.13. I would appreciate it if Mojang took this into consideration if they decide to revamp commands again in the future.