PaperPrincess has a major point......a design wall works MUCH better for you if the fabric on the wall is a 100% cotton -- either as a batting, flannel, or other "sticky" cotton. Cottons have an affinity for each other, that's why we use them like this. Felt is more often a synthetic than a cotton.
I, personally, would never use a felt, much less a green felt, on my design wall. (Green just conflicts with everything I'd want to audition my blocks for!!
)
Jan in VA