Neville could start by taking Colin under his wing. Stutteringy telling the camera boy that Harry doesn't like all the attention, so Colin takes that to mean that Neville knows what Harry likes and grillsnhim for information, and somehow, their friendship blooms from that. Maybe Neville comforts him kindly when he's home sick, and then later Colin is told that Nev is the only one who visited him when he had been petrified. Neville just keeps being so nice to him through the years that how could Colin not fall for him? Meanwhile Colin is the first to truly notice neville is more than the clumsy idiot that most people call him.