Scott made the games and said he was blue, bonnie looks purple because of the lighting in fnaf 1 and 2, probably hinting to why bon bon is blue in sister location because he is glossy and in light lighting, other than OG bonnie, who is fuzzy and in dark lighting.
Rockstar bonnie is also blue, and it's because he's glossy and in light lighting, and it's easier to see the colors with those circumstances
When Scott made bonnie, he wanted him to be blue, and made him blue, how do I know that?
It's because Scott himself even confirmed bonnie is blue