I'm sure you've all heard of the famed Green Glass Door:
A butterfly can go through the door, but a moth can't.
A sheep can go through the door, but a horse can't.
etc.
However, I have found a door of a different color!
A wasp can go through the door, but a bee can't.
An orange can go through the door, but an apple can't.
A soldier can go through the door, but a sailor can't.
A ninja can go through the door, but a gladiator can't.
Silver can go through the door, but gold can't.
What is the obvious choice for the color of this door? Why?
Any colors directly mentioned in this puzzle are not eligible to be the color of the door.