This issue first appeared in 24w11a when the changes to effect particles were made.
The particles created by a creeper when it explodes with effects applied will not use a collection of colors, but instead

Steps to Reproduce:

1. Spawn a creeper
2. Apply numerous effect particles to it (recommended to use distinctly different colors)
3. Light the creeper with flint and steel

Observed Behavior:

The particles created will be an average blend of all the colors of the effects applied.

Expected Result:

The particles would all be uniquely colored, randomly selected from the effects which were applied.

Screenshots/Videos:

Notice how the particles created on the creeper and myself are random colors, yet the particles created after the creeper explodes are an off-gray color.
EffectParticles.mp4

Notes:

Related to: MC-269994