UnboundLocalError: local variable 'colorIndex' referenced

[silverburgh.meryl at gmail.com]

Can you please tell me what is the meaning this error in general?

 UnboundLocalError: local variable 'colorIndex' referenced before
assignment

In my python script,
I have a variable define and init to 0, like this
colorIndex  = 0

and in one of my functions,  I increment it by 1
def myFunc 
   colorIndex += 1