Many will know this dilemma in a variety of guises. You've assembled the miniatures you want. You've undercoated them black (or grey, or white). And now you're looking at it seriously. What colour do I want to paint my forces? I mean, we all thought we liked red a lot. But then blue sounded excellent. Some of us even thought yellow would be neat.
I faced this issue myself with my Warhound Titan.
To decide what colour it would be, I turned to the background. I want my titan to belong to the Legio Perennia. Now, this is a titan legion that very little is known about in terms of heraldry or colours. I kind of like this as it leaves the door wide open to what colours it could be painted in. Simultaneously, it provides a dilemma. What colour do I want? Why can't I decide?
Then I thought about the name "Perennia". To me, it has overtones of "perennial". As in the plants. You know, perennials are the type of plant that live for more than 2 years. And of them, there is a subset -- the evergreens -- that are ever green. Green!
And that was the extent of my logic that decided the base colour. And besides, apart from Death Guard in 40k, I've not really worked with green too extensively (at least in a neat, clean and not Nurgle's Rot kind of manner).
Turning to the air compressor, I applied a layer of Vallejo green to the black undercoated and silver highlighted base layer. The green layer is thin enough that the silver highlights I had previously sprayed on can be seen through the green layer, creating a metallic green appearance and hints of how the metal pieces have been bent in to shape during manufacturing! Or at least that's what I'm trying to communicate here. Hope you like it and the effect!