Mixing Bettas
All juvenile female bettas have horizontal stripes. As they mature, the stripes tend to be less evident or get lost in the overlay of the colored scales. (This does not apply to the Cambodian Bettas as they have no natural stripes.)
The female in this picture shows the vertical stripes present when the female is in breeding colors. She turned this color pattern on within 5 minutes of her seeing her future mate.
Georgieb45, If you are not seeing this pattern on your female, she is not ready to breed and shouldn't be with the male if you are serious about breeding them. It most likely means that she is too young (based on your picture) and if the male gets ready but she's not, you could miss the opportunity to remove her because the male will kill her.
Also, as recommended, read up on breeding bettas because these fish are not community tank breeders and rarely breed in a tank with "normal" water circulation. All you appear to be doing is putting the female in jeopardy.