What might you mean by “too bright”? Do you mean that the surface finish is too shiny, or do you mean that the finished paint surface is a brighter shade of the color you originally wanted?
Are you trying to tone down paint that’s already been applied?
Once the color has been applied there’s little, if anything, you can do to lighten it. Darken or subdue it a bit, possibly.But not lighten. I’m assuming you’re wanting to apply a thin coat of white to try and make the colors ‘less bright’? That really won’t work. It’s not like photo editing…
You can use as many coats of paint as you want. I would suggest several light coats. If you apply the paint too heavy then you run the risk of sags, runs and obscuring surface details.
Clear flat, like Testor’s dullcoat may knock the intensity down a bit. But not much.
Short of repainting there’s not a whole lot you can do…
You can use chalk to weather the paint you’ve applied. I would do that after the decals are applied and sealed in with flat finish. If the color is too light, you can use a dark color (or powdered charcoal) to darken it. If it’s too dark, you can use a lighter shade of the same colored chalk to lighten it a little.
I use Scale Coat II regularly. You can tone down the high gloss finish with either acrylic or solvent based clear coats; semi-gloss or flat. It’s difficult to make a reasonable recommendation without viewing the work.
Can you take photos of your unit and post it here?