For some reason I have a tendency to gravitate towards marbles...I have NO idea why. I'm not -consciously- partial to them(not that I don't find them lovely, but I don't prefer them to other colors or anything xD), but they seem to be the ones that 'call' to me the most out of any other color pattern. I can't remember a time when I didn't have at least one marble around. I even once had a 'surprise marble'....a lovely dark Orchid/red boy who suddenly started getting lighter marbley patches on him. xD
It is indeed quite fun to watch them change over time though. I find they do indeed get more solid/darker color-wise as they age, though occasionally they'll 'marble backwards', I suppose, and get a lighter patch here and there.
Currently I have two marbles(as far as I know, still trying to figure out Finni....though with my history he probably is one xD), Ciel and Dice. Ciel I bought 7 months ago tomorrow looking like this
, and now he looks like this
. Dice is the lovely boy in my Avvy, I've had him for almost a month and no significant changes yet, but I REALLY look forward to seeing how he marbles ;)