I used black since it help create some natural shading in recessed areas when covering with the grey paint
Tamiya is hard to brush paint over large areas. I find it works nicely for small touchups in areas but it is really hard to brush paint smoothly something that big
You would do better hand painting with Vallejo or even better Privateer Press P3 paints and use a larger brush
Something similar to the "Flat" one in this picture
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Your best bet would be to find a spray paint of a similar color
I did my larger Revell AT-AT with Testors Light Aircraft Grey. Being Testors enamel though it took weeks to fully dry without feeling tacky. It dried to the touch within and hour, but felt slightly sticky when handling it for weeks.
Maybe the Model Master line has something similar
Of course another good alternative is just use Testors Dullcoat on the model, then weather it with some oil washes and pastels and skip painting all together. The Bandai plastic color after all is a very good color to start with
The dull coat will take away the plastic sheen as well as provide a bit of grit for the oils and pastels to cling to