As I understand it, there is no physical risk to having your hair dyed anytime you like. However, some cosmetologist's recommend waiting for a year because your hair color and texture can continue to change for that long and that can affect how it takes the color and how it looks.
Personal, I absolutely love my post chemo hair. I love the color and waviness and softness of it. It feels so much healthier and less dried out than it did pre chemo when it had dealt with years of coloring and other hair products. Now I just shampoo and it's good to go.