If you're asking why he doesn't look anything like Gyarados: The first part of this video explains it. http://youtu.be/aftr_dWrht8 If you're asking why he's orange, you know, just in general: Why not? Orange is pretty legit. Plus he's more of a reddish tint.

They used to come in many colours. But then an orange supremacist took over and ordered all non-orange magikarp to be exterminated. They tried to fight back, but the only move they knew was splash...

Yo listen up, here's a story About a little guy that lives in an orange world And all day and all night And everything he sees Is just orange like him Inside and outside Orange his house With an orange little window And an orange corvette And everything is orange for him And his self And everybody around 'Cause he ain't got nobody to listen... I'm orange, da ba de, da ba die Da ba de, da ba die Da ba de, da ba die Da ba de, da ba die Da ba de, da ba die Da ba de, da ba die Da ba de, da ba die

Because they only eat orange skittles.