Coloration Question On My Koi Angel

[trojannemo]

hi there.
i've currently got two angels together. due to their young age i haven't been able to sex them, if you could that'd be great!
anyways, they get along great, very few times i've seem them bicker in the last 3 months.

now i started to notice on my Koi Angel (or so i thought when i bought it much smaller) a new coloration he didnt have at first.
this is he/she:

if you look at the back half of his body in that pic, there's nothing of interest there. well lately
i've noticed that while the lights are out, the back turns BLACK just like a half-black angel. more recently he's been pulsating that with the lights on...but very briefly. then he/she goes back to looking like in the picture and goes about its business
here's a quick picture:

i'm pretty confident it's nothing for me to worry over, but i'm curious as to why he's doing that. is it reaching sexual maturity? is it now getting in it's half-black strain (koi/half black mix?)

thanks for any light you can bring to this situation

EDIT: here's a quick video i shot of it going from dark to clear on the back and then back to black again when i turn off the lights. check it out tell me what you think.
right-click save as

 

[Goodygumdrops]

Tbh,I'm not sure I'd call him a koi,he doesn't seem to have any orange.Be interesting to see what he develops into once mature.

 

[trojannemo]

Tbh,I'm not sure I'd call him a koi,he doesn't seem to have any orange.Be interesting to see what he develops into once mature.

aww dont say that! i was so intent on getting a Koi angel to go with my silver zebra...but the worst of me caught up with me when i saw him and purchased him...and now i'm Koi-less

when you say "he" are you just being general or would you say he's a male? i know sexing these guys while immature is difficult but just wondering if anyone sees any tell-tale signs on him yet

 

[Goodygumdrops]

Sorry,I was just saying 'he' probably because you did (subliminal and all that).It's next to impossible to sex angels until they pair up and start breeding.
Any pairs developing yet?

 

[trojannemo]

Sorry,I was just saying 'he' probably because you did (subliminal and all that).It's next to impossible to sex angels until they pair up and start breeding.
Any pairs developing yet?

no pairing and i doubt it'll happen. i only keep 2 angels at a time with my rainbows. this is my second "pair" per se, since the first were wiped out together with my rainbows when some monster disease came in and the rest is a tragic story

dont really intend to breed them or anything. just curious about their sex...but i'm fine, they can have a "partnership" if they're same sex

 

[Goodygumdrops]

Well yeh,if you're broadminded,where's the harm?