I expected the new CustomPotionColor tag to not have cases where it reveals its underlying non-custom color.

The effect particles that players/mobs have are intended to have the non-custom color (see https://bugs.mojang.com/browse/MC-108943?focusedCommentId=348605&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-348605)
The particles coming from players/mobs are unchanged.

Steps to reproduce:

1. /give @p potion 1 0 CustomPotionColor:0,Potion:leaping}
2. Drink; notice that your particles are not black.

1. Drink milk.
2. /give @p splash_potion 1 0 CustomPotionColor:0,Potion:leaping}
3. Throw it on yourself; notice your particles are not black.

1. Drink milk.
2. /give @p lingering_potion 1 0 CustomPotionColor:0,Potion:leaping}
3. Throw it on yourself; notice your particles are not black.

1. Drink milk.
2. /give @p tipped_arrow 1 0 CustomPotionColor:0,Potion:leaping}
3. Fire it so it hits you; notice your particles are not black.

The area-of-effect cloud's color is unchanged.

Steps to reproduce:

1. /give @p lingering_potion 1 0 {CustomPotionColor:0,Potion:leaping}
2. Throw it; notice the cloud's color is not black.

The particles coming from a tipped arrow are unchanged, and when that arrow is retrieved, the custom color is lost.

Steps to reproduce:

1. /give @p tipped_arrow 1 0 {CustomPotionColor:0,Potion:leaping}
2. Fire the arrow, notice the particles are not black.
3. Retrieve the arrow, notice it is no longer black.

This is because the arrow and ActiveEffects tag doesn't save the CustomPotionColor tag, and the effect cloud doesn't write it to Color.