My stash building is only what is on sale.
Mostly I collect background fabrics, batiks, and backing.
These I know I will use.
My favorite colors only if I LOVE the fabric and it is useable for many different blocks (small prints).
The colors have to be very adaptable to other colors, like basic greens and blues and roses, odd shades of colors are not good choices for a stash.
I also collect red, white, and blue for charity quilts.