Between you, me, and the gatepost, my guess is fabric companies provide patterns for quilts and then the magazine uses that company's fabric for the instructions - or the magazine makes the pattern and has a deal with the fabric companies..
That said, I have never made a magazine quilt with the fabric called for in the pattern - most of the fabrics called for are $$$ stuff at the high priced LQS and I don't go there.... LOL
I either use out of my stash (as I've said before, I usually by three to four fabrics as a 'quilt' - with no pattern in mind) or I might, if the pattern really jumps out at me, buy fabric with the pattern in mind....
Short answer, no, I don't use the fabric called for - and I'll use graph paper and colored pencils to figure out my colors.