Depending on the design, and colors in it, you may be able to use paper for lights on the heather gray. For the sport gray, I would use an opaque paper for darks.
My paper of choice for lights is
JPSS (Jet Pro Soft Stretch). For darks, I like Ironall for darks (aka Everlast for darks). Both of these are available at
www.heatpressessentials.com.