Depends a lot on the look you want, but watercolors are by far the easiest to use (and I personally love the look of watercolors!). You also have the options of acrylics and oil paint, if you want darker, richer colors. You won't be able to see much of the pen lines through either of those, though, while it's very easy to control the transparency of watercolors. Those are my two cents Color pencils and pens can be used to amazing results too, but I have no experience with that, so I can't say much about it

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