6 months is the average age of being sexually mature. The only reason a female would start digging is to make a nest. Not all females will display the blue colors you are looking for BTW. If you want to know if she's ready for breeding, but the male and female together and observe her reactions. If she turns a very dark, almost black color, hisses alot, and rocks back and forth alot when the male comes around, she is not ready for breeding. Also, you say she is digging in her substrate. If she IS building a nest to lay eggs (which is prolly what she is doing) make sure you give you female a laying box rather than just using the substrate. It's easier for you to get the eggs, and with a box, the female usually will choose the box to lay because it is less stressful for the chameleon. Hope this helped!!