How do I verify that this will be random rgb value

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Python

Novice

The whole thing really is what im wondering about, first time doing this without for with for and so on generating all combinations.

"""Dont know how to check for errors numerically because of to many iterations
or how to check by derivation proof
"""

from random import randrange

"""Random rgb color combination
0<=red<=255
0<=green<=255
0<=blue<=255
Supposed to be all combinations of integers from span above
255**3
"""
def Random_Rgb():
    a = randrange(0,255**3+1,1)
    red=0
    green=0
    blue=0
    #One red cycle
    while a >= 255**2:
        a -= 255**2 
        red += 1
    #Green cycles
    while a >= 255:
        a -= 255
        green += 1
    green += (red*255)%255
    #Blue cycles
    while a >= 1:
        a -= 1
        blue += 1
    blue += (red*255*255)%255 + (green*255)%255
    return red, green, blue

red, green, blue = Random_Rgb()
print(red,green,blue)

What I have tried:

Numerically: wrapping in for, too many iterations
Proof: I barely understand combinatorics: The form on wich its stated is confusing to me also too much text in online guides I loose track so instead i tinkered with combinatorics but i dont feel the least bit sure the code is right