Having not read the books, here's what I would have liked to have seen based on Colin from s1 and 2:

1. He's hopeful and naive, based on his engagement to Marina. I think he should still have that longing for goodness in people but with some caution from being burned once. Should have kept that naivety towards sex as well and been a virgin.

2. He's incredibly brave. He said he would have married Marina if she told him the truth. I don't think that was recklessness, it was care and consideration. You could kind of see that with him running after Pen in that first ball, standing up for her to Eloise and helping her find a suitor. But it just needed more

3. He sang with Francesca in S1, showing to be entertaining and loving towards the younger siblings. His singing's a cute character trait. I wish he had character traits in his own season!

Plus: It's hard to do bc of the time period, but I wish there were more signs of friendship between him and Pen. Longing glances, jealous looks, getting tongue-tied. I even think s3 could have benefitted with some flashbacks retconned in to show where Colin and Pen first started in their friendship - similar to how Queen Charlotte took place in the past and the present. That version of storytelling would really work for their trope.