Not pure bred Black Moscow Guppy

[Mayanearth]

Hello forum, I have 10 "pure bred" Black Moscow Guppys, but two days ago I noticed that my "best" male has some yellow on its upper fin. I got them at the ACA meet in Indianapolis and was assured that they were pure breed. I am wondering if I should let him breed with the other female Guppys.

I would really appreciate it if those of you are knowledgable on this could give me some advice.
Thank you all.

 

[Samzter]

Well, I wouldn't only because it would ruin the strain. You can if you want because it won't do any harm.

 

[Mayanearth]

I think I will let him mate with the female Guppys and see what his children look like.