if i had to guess, i`d say the gray stuff is rip stop w/ a DWR finish. if it`s like what i found there, it`s great for quilts, but i don`t know about spraying it w/ silicon & trusting it to be water proof.
you didn`t say if the camo you plan to make a tarp out of has any treatment that can be seen.
of course you said that you were just making a tarp for practice.
don`t leave the CREATOR out of the creation!