I just built a Gundam kit.

It's the original Gundam from the 79 show, RX-78-2. The story goes that the director wanted it to be a serious show, but was forced to make the Gundam white with the blue, yellow, and red colours by his bosses, so they could market it.

Looking at it now, I could rationalize it as a prototype, where the various important parts are colour-coded by the design team to show off their functions to the Brass.
As a prototype weapon pushed into combat, I guess it fits. And in doing so, the Gundam's look and colours went down in history as a symbol, so we'll never get a gunmetal gray Gundam.

Also