It doesn't matter what Potion you equip or whether you used the CustomPotionColor Tag, when I equip an Armor Stand with Potions, they appear in a Magenta Color strictly.

Example in Picture: I tried to make the shown Armor Stand hold Water Bottles, but unexpectedly, the Water becomes Magenta when rendered. I tried changing the Color to Blue (#0000FF) but still, Magenta!

Update: Some cases DO have the colors work well, but the case in this Picture must have something wrong. I'll investigate further...