Actually I'm an Art Major and I just happened to stumble on the color two days after I bought him. It's not a pigment or anything but just one of those colors thrown around on the internet, it's similar to a light cerulean or almost a sky blue
And for colors, well if it's not a marble, then yes colors usually start to develop around 9-ish weeks and stablize through maturity although some don't get their full colors right up to 6 months. And then Marble genes can change at any given moment, so they could be a year and a half old and still be changing colors! Which I find to be absolutely fantastic and exciting! But then sometimes it's sad to see their original colors leave, you know? You bought them as one thing, really liking it and then they just completely change on ya.
Ah yes, I like the regular rectangle ones ^_^ I've got a Top Fin 5.5 myself and love it! It's housing my two baby Betta's at the moment
but yes, all my tanks are planted and I love it! I'm working on my lighting systems at the moment but so far so good! Low light plants would be just fine for you if you got them, I can coach you through plants and stuff if you ever wanted to, just shoot me a PM whenever if you do!