It's an afternoon wedding. That it might continue past 6PM doesn't change that fact. Yeah, an afternoon wedding that starts at 5:30 might be thought of as an evening wedding. But not a 3PM wedding.
Wear your cotton khaki suit, if you wish. Or wear a more formal wool suit, if you like. But do not wear the cotton suit for the first part, then change into a more formal suit for the latter part. That's an example of worrying too much about what to wear.
Since the invite specifies "cocktail attire," I'd probably wear a "real" suit, and not the cotton khaki one. But that's me. If I saw someone at the wedding who was wearing a cotton khaki suit, I wouldn't automatically judge him to be an uncultured barbarian. "Cocktail attire" is one of those dress codes which takes in a lot of ground.
If you have friends and family members attending this wedding (typical, for a wedding), you might want to take your cue from them. If all of them mention to you that they're going to be wearing charcoal (or navy) suits, well, perhaps you'd be more comfortable joining the crowd. Just a thought.
Thanks for the input, especially the bolded. I think I needed someone to tell me that. I feel like I am going to feel out of place in the evening in a light colored suit, so will probably go with the dark suit for the whole day.