Both the item and the entity model applied of horse armor will have the entirety of their textures colored, including parts which should not be considering they already have (non-white) color applied.
This results in a muddied texture where sections become visually unappealing and unexpected to the player using it.

This can be specifically demonstrated when compared to the Bedrock edition which does not have this issue. The texture appears to be colored correctly, with all non-white pixels being left alone.

Steps to Reproduce:

1. Summon a tamed horse:

   /summon horse ~ ~ ~ {Tame:1b}
   

2. Get a horse armor item, and apply it to a horse
   → observe how even when not dyed, the item and the entity model texture both have a discoloration to them, primarily observable on the saddle.
3. Dye the item lime green (for comparison) and equip it onto the horse

Expected Result:

Both the armor item and entity model textures would have a tint applied only to the sections which are white in the base texture, or sections which do not have color alterations made unique to the texture.

Comparing this to Bedrock, this would be the expected result:
(Notice how the saddle and reins are left alone, while the rest of the texture is lime green)

Observed Behavior:

Sections of both textures will have a tint applied to the entire texture, making everything look muddied and unpleasant.

Notes:

More recently as of 8/1/2024 MC-16829 has been reopened, which is another related issue to do with horse armor not appearing visually correct with the enchantment glint and may be relevant to this report.