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Acrylic paints are your friend here. They last a long while in their bottles and you can keep a palette for a few days when using a wet palette. start with just using a brush and jump up to an airbrush if you want to cover a bigger area or do fancy stuff (an air brush is far from required). As another commenter said, the minipainting community has a ton of resources, text and video tutorials, and willing to provide constructive feedback if you want it.
Just remember: thin your paints~