some ppl: Colin started to love Penelope after the kiss.
my galaxy brain: It was actually, when she stopped replying his letters!
She 'changed´ first and subconsciously contributed to his identity crisis because he felt lonelier than ever. He was used to his family not replying, but not Pen.
When he comebacks he's already obsessed. Penelope is just minding her own business and here comes Colin Bridgerton whenever and wherever she is, trying to speak to her and just be with her like a lovesick puppy (Penelope minding her own business in her own garden and here comes Colin Bridgerton out of nowhere like 'boo, surprise!'). The moment he learns he fucked up last season, immediately offers his help to make it up to her (which, in my mind is already an unconscious love confession idc). He already feels unsure the minute Pen gets attention even before the kiss!
The kiss was just the catharsis of his already growing feelings. And even the kiss is interesting because he accepted it, unlike when he couldn't kiss Marina in S1, but he did could kiss Pen. Partly because, unlike Marina, Pen was very honest with him, showing her completely vulnerability and on the other hand, perhaps, a kiss doesn't seem like a great deal now that he was more experience. And partly because the idea sounds rather good than her, so it's a bit of pity and a bit of desire. BUT BOY DID HE PLAY HIMSELF. Colin's feeling had been growing and growing and the kiss was like a running face into a wall; awakening. The connection he had been searching for during his travels he finally found (after he lost it when Pen stopped replying).
But it was not the reason of his feelings turning romantic.
My point is, Colin would have come to realize he loved Penelope romantically even if they hadn't kiss. It was already there, blooming slowly. The kiss just helped him realize sooner, rather than later when it was already too late.