Previously it was possible to spawn entity_effect particles that had random colors. The behavior where <delta> was interpreted as RGB only worked when <count> was 0, so for any other values the command produced random colors. For example the following 1.20.4 command:

/particle minecraft:entity_effect ~ ~ ~ 0.2 0.3 0.2 0.5 32

In 1.20.5, the entity_effect particle has a required color field. Existing commands are unable to be upgraded.

/particle minecraft:entity_effect{color:[0.2,0.3,0.2,0.5]} ~ ~ ~ 0 0 0 1 32