生成器运行中途停止并抛出IndexError问题求助

我正在尝试编写两个生成器，用来为Pygame生成RGB值，其中一个生成器会调用另一个。生成的数值需要在0到255的范围内浮动（先增长一段时间，再下降一段时间）。目前代码能运行随机步数，但总会在某个随机时间抛出 IndexError: list index out of range 错误，请问问题出在哪里？

from random import randint as r


#this generator generates two lists of values to float back and forth between 0 and 255 (supposed rgb value) 
def func():
    a=r(0,255)
    b=r(a,255)
    
    while True:
        yield list(range(a,b))
        a=b
        b=r(0,a)
        yield list(range(b,a))[::-1]
        a=b
        b=r(a,255)

red_iter=func()
green_iter=func()
blue_iter=func()

# this generator yields one value per its call from lists generated by above func() generator 
def shifter():
    red_counter=0
    red_range=next(red_iter)
    while True:
        print(red_range[red_counter])
        red_counter+=1
        if red_counter>=len(red_range):
            red_counter=0
            red_range=next(red_iter)
            

while True:
    print(shifter())

备注：内容来源于stack exchange，提问作者Victor Dmitriev