Armour stands (as well as item frames and similar entities) change colour and lighting as you move around them. Leather armour loses all colour.

1. Place Armor Stand
2. Place any armor on the Stand
3. Move around and any entities present will start changing their light level and colour

In the last 2 screens you can see 2 Iron armours with different lighting as well as one Item frame which is illuminated as if from every direction. On the first of those two the item frame is not illuminated.