I've used pigments for a couple of months now, and they're great for weathering. As mentioned above, another alternative is artists pastel chalks. They are basically the same thing, however a set of colours is quite pricey.
I've made washes with them and used them straight as they are, but I'd not recommend using pigments as a paste for mud, as it's a waste of pigment. Better you use the pigments to colour and dust the mud, but as for the actual dirty stuff itself, use something like Mr. Surfacer, or just sift fine dirt and make real mud
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