I know Gundam when I was a kid, it's cool. I bought a few Gundam kits, but most of them are still sitting in the boxes. This one, a Master Grade or MG series of the ordinary RX-78 Gundam. After made a lot of car models, Gundam modeling do a challenge to me. This is now Youtube comes in handy. There are so many tutorials about Gundam modelling.
This is how they do and I tried on mine too. Firstly, after grey and white primer, I applied a coat of sand yellow and white mixture (in 1:1 ratio) on those suppose to be white parts. This is one way to imitate the dirt or the old marks around the machines. By doing this an airbrush is a must. Once the sand yellow/white dry, I airbrushed white onto it, but not covering the whole parts, just covering the middle and leave the edge a shade of sand yellow. For the red parts, I applied red all over, then lighten the paint up with a bit white. With the same technique, spray only on the middle, leave the edge a darker shade. And applied the same technique on the blue parts.