Starting out I would use one of the already proven inks or watercolor in tubes.

With carbon printing it is extremely important to limit your variables to ensure some kind of repeatable results.

If you have any interest in varying your picture "temperature" (i.e. cold, medium, warm depending on the content of the image), I would definitely start (and stick with) watercolors. The pigments have been specifically developed and processed to dissolve consistently in water ... and, you have a product-wide line of different colors that have been developed by the same company for mixing. Unlike inks, the compatible choices are myriad and they are readily available at art stores everywhere.