Painting a model has several purposes. The first is to dissipate the plastic look. It hides the stress mark and all the little flaws on the parts. You can respect the original color scheme. On the assembly guide, there is always a section about what type of colors you should use to respect the original color scheme.
You could decide to change the color scheme. You can decide to switch colors, or you can do something totally different. I'm used to trying my color scheme on photoshop. I download a picture of the Gundam I would like to customize, and I test different colors and pattern.
The difficult part is to find a color in photoshop that will look the same than the paint you have. Otherwise, you will have a nice picture of a custom color scheme, but no way to reproduce it in your models.
That's why you need paints, a lot of paints, and even more than that.
You don't want all your Gundams look the same, so you will have to change the hue, the shade and the intensity of the colors.
One thing I have discovered is when you paint a small panel on a part with the same color but a bit darker, it always adds more prestige to the Gundam.
At some point, you will want to add more paint details. So you will need metallic colors or transparent colors or...