Program: shirt order

[Truman]

color = input("Choose color between white and blue: ")
size = input("Choose size: ")
available = False
while not available:
    if color == "white":
        if size == "M" or size == "L":
            print("available")
            available = True
           
        else:
            print("unavailable")
            break
    elif color == "blue":
        if size == "M" or size == "S":
            print("available")
            available = True
        else:
            print("unavailable")
            break
    else:
        print("Please choose the correct color!")
        break

I don't understand why Python tags don't work now.
Anyway, I have several questions regarding the code now.
1. this part

available = False
while not available:

"while not available" in this case means True since we have "available = False", right? I still don't understand why we need this in exactly this way? Can we do the opposite? Set available to True...

2. When adding some third color I first receive Choose size input. Is there any way to avoid this? To get "Please choose the correct color!" immediately if I, for example, type in green colour?