Infinite loop when running it in a function

i am new to python and i am trying the following code. The problem is that even after one of the conditions are met in the if statment and a is set to false it goes back to the beginning of the function setting a to true therefore never ending.

def secColor():
    a = True
    while a:       
        c1 = input("Enter the first color (red,blue,yellow)").lower()
        c2 = input("Enter the second color (red,blue,yellow)").lower()
        if c1 == "red" and c2 == "blue" or c1 == "blue" and c2 == "red":
            print("Purple")
            a = False
        elif c1 == "red" and c2 == "yellow" or c1 == "yellow" and c2 == "red":
            print("Orange")
            a = False
        elif c1 == "blue" and c2 == "yellow" or c1 == "yellow" and c2 == "blue":
            print("Green") 
            a = False      
        elif c1 == c2: 
            print("Error! You must enter two different colors")                    
        else:
            print("Error! The color must be one of the following (red,blue,yellow)")

secColor()