I bought an OptiFine cape few months ago and I saw a player wearing an OptiFine cape with custom (black - #000000) background with white skull symbol.
How? I checked NameMC and it is an OptiFine cape (not lunar client cape). I will not share the actual player name, if it's not needed.
As you can see from the images, the background of that background is completely black but I think OptiFine doesn't allow custom colors. Am I wrong? Please correct me and if you know how to achieve this or a similar cape, I would be happy if you could share it with me.