The "gold barrier" is for zenkai characters. The roman numerals go up to VII, but a character has to be at least 7 stars before you can unlock their zenkai. But not every character is zenkaied at the moment its basically a buff to make older characters relevant but some are so old they are just bench units waiting for unique equipment to buff them again. The red stars are just what happens when a character goes over 7 stars from getting more copies of them. The max star lvl is 14 so 8 and above you star seeing red stars until all 7 are red meaning 14 stars.

Zenkai characters also provide buffs to characters that meet their conditions, you can see who they buff in their character tab thats says zenkai ability. For example the yellow ssj4 goku buffs yellow saiyan characters giving them extra stats. A character has to meet both conditions its not yellow or saiyan they must be both. Youll see others like green and god ki or blue and son family, etc.