Hey Ty
the worst thing about PVC is if it puckers or wrinckles they are there to stay, nylon is always shrinkable.
I don't know about the earth pigments but it sounds like a fun experiment to try on some small scraps..but I would think any water soluble pigment should work as long as you don't add too much, let us know how your samples work out.
you really don't need a $200 cutter/sealer. an electric soldering iron works fine,a butane one works better.you can use a utilty knife blade held in a pair of vise grips,get the blade good and hot with a torch and make a quick cut..or you can cut the nylon with a sharp pair of sissors and then use a torch to melt the edge,those cheap soldering torches with the refilable lighter inside work well for this method cuz they have a small easily controlable flame.
save you 200 bucks...crap,thats almost enough to build another boat. canoes and kayaks are like footware..you need many different kinds.LOL :-)