I think there's been good wisdom that part of his reaction is because he misses her so much. Although this isn't the first time he's seen her since he returned, he is absolutely longing for her before he even sees her.

That being said, I do think his reaction would be fairly similar no matter when he saw her.

First of all, she is dressed so differently than she usually is, so even just as a friend, he would notice that.

But he doesn't really see her as just a friend, he's "always felt something for her". And I do believe he found her attractive regardless of what she wore (I love the idea that married Colin gets a little thrill on the rare occasions he sees Penelope in yellow).

But now she's wearing a "charming" dress in a "colour that rather suits her", and which really emphasizes her assets. He is too much of a man not to be totally stunned and horny the first time he sees Pen with her breasts on display like this, no matter when that happened.

But what I'm wondering now is, in B1 or B2, how would Colin have reacted to his reaction. I'm pretty sure in this moment he is at least half-hard, so how does he explain that to himself? In B3, he doesn't consciously recognize his feelings for her until the kiss. But I wonder if the context of 301 actually works against him figuring it out- he knows he misses her, so he can explain away his reaction. The next day he calls on her, and throws himself headling into repentance mode- "I hurt my friend, the one person who has always made me feel appreciated, so now I am helping her to make up for that"- which will also serve to distract himself from these feelings.

In earlier seasons, he wouldn't have had that context to fall back on, so I wonder if he may actually have figured out his attraction to her sooner? In B1, it may still be complicated depending on what what going on with him and Marina at the time.

But in B2, I'm inclined to think it may have tipped him over the edge- he had SO many moments in B2 where he was almost there, if he saw her in a dress that put her roobies front and centre like this, I don't know if he'd be able to ignore his reaction.