According to most of the appearances of Bonnie He´s actually Blue, but not the exactly Blue you would think.

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It´s a Mix of blue/purple?

Not actually, the color of Bonnie it´s part of the blue scale, however it gave the illusion of being some kind of purple because it´s in the middle of the saturation, where white values makes the color more "soft" and less intense.

At the first moment we don´t see that color blue, but look the actual same saturation position in the purple scale.

Now let´s compare:

(very similar, but not the same)

Now let´s look Bonnie from FNaF 1 Renderized and without the purple light of the showstage

Now we can see more and more differences.

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But Why we see Bonnie of Fnaf 1 with that Illusion?

The saturation of the color it´s not the only reason, the textures that Scott uses on the animatronics have darker colors, that makes the illusion more stronger.

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Ok, but what about things like this?

In this officially art we see that Bonnie is purple not Blue

That´s actually true, and you´re not wrong, but I think there´s two big explanations:

1. This is because the draws and pictures of the character have that colors on purpose.

Something like errors that they known but they don´t mind.

Foxy from Freddy and Friends it´s not the same red as the original Foxy and it´s a fazbear cartoon (mistakes like that).

2. Like the fandom, they had been mistaken him too.

The fandom it´s pretty big, during years people draw Bonnie in purple colors (which it´s not bad),and if you search you will see that Bonnie with purple colors only had happen when scott put others in charge, like steel wool and the FNaF official artist´s.

Even more, search between Fnaf 1 to FFPS (Fnaf 6)
and you´ll se that Bonnie is blue and never purple!

Sprites from fnaf 2

Sprites from fnaf 4

This is probably the actual reason, people exterior to scott draw Bonnie with purple colors when he´s actually blue!, a little error which it´s ok.

So hey, what do you think about this? tell me in the comments!

-Mr.Hags