Design #1: Fall guy
As cruel as it sounds, Colton originally was designed as a way to give Darkpenny (the story he was introduced in) some weight as, quite literally, the character who lost out. Stemming from a misplaced notion that stories gain weight and relevance as a story through loss (really, it's through change), the thought was that Colton would be wanting Penny as much as the trio and would be left out in the cold when they reunited.
While it worked, it was also a bit...mean-spirited. While there were no immediate plans to bring Colton into the Pennyverse proper, we decided within two days he needed a happy ending as well.
As far as visual design goes, Colton really hasn't changed that much since Caby initially drew him in 2019. His slight blue coloring was intended as a part of a visual theme, where those ultimately meant for Apricot Bay would have stranger colors than City folk. About the only visual change he undergoes throughout the story other than his clothes is his proportions; Colton's rather malnourished up until the Art Gallery, and certainly prior to moving in with Madeleine's family, so he becomes a bit fluffier and certainly less bony in the process.
Design #2: Spooked little kid
Pretty much everything about Colton has either changed or been up in the air throughout his development. I could spend all day listing various plot threads for how he meets people, various character traits he went through, all long abandoned. For the sake of my own embarassment, I'll keep it to what was actually published.
It was decided really early on that Colton would have music as a way of working through his issues, and Riley became both his mentor and the guy who gives him his first guitar. This would also become the way he meets Madeleine, ultimately. A huge chunk of his arc was meant to be spent trying his best to get Penny's forgiveness and work around Sebastian anger after the events of Darkpenny, but when said events were downplayed (and the weird grabby assault retconned), this too was downplayed.
Colton used to be a lot more nervy and scared around strangers. This didn't work for two reasons, the first being that an avoidant character makes it rather difficult to push a plot forward, but also that it didn't quite fit a young growing boy with hormones and a need to prove himself going on. While we still loved him, there was something never quite right with his personality, and boy did it bug me.
Design #3: Confident, independent, comfy
Finally, nearly two years after his creation, Colton got a pretty massive overhauls in his personality and plotline, bringing him to where he is today. I had an epiphany a bit after "Gonzo the Dissident" was written; Colton seemed far more comfortable, and not to mention writing him came more naturally, when he was instead depicted as a go-getter, independent, resourceful sort of lad like how he appeared in the story.
With the guilt from the events of Darkpenny dropped, Colton had no more need to be as timid and shy as he previously was even before the events of that story were supposed to take place (within reason, kids and teenagers are always a bit weird around strangers). Thus, instead, the reason Colton came to Apricot Bay changed from "wanting to make up with Penny" to "wanting to start over for himself in a better place". Both in his good moments and his bad, it just made more sense, not to mention was a lot more interesting to read about and made for a better protagonist.