[Python] Generators stop working in the middle of process

I'm trying to make a 2 generators which yield rgb value for pygame. One generator is callin another one. The generated value should float (increase for a while and then decrease for a while) within 0 and 255 limits. The code working for a random number of steps but then at random time brings IndexError: list index out of range. What is wrong?

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())


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