Here's an easy idea to make a translucent window with some color.
I took a piece of Heat Resistant Vellum and embossed the mushrooms from the Mushroom Forest Set onto the vellum with Gilded Embossing Powder. I set up the panel in my Misti to fill the entire panel with the little mushroom stamps (moving and stamping several times). Once heat set, I flipped over the panel and colored the mushrooms with alcohol markers on the underside. I used red, orange and brown alcohol markers.
For the actual card, I made a top folding A2 sized card, cut off one inch from the bottom of the top panel, and then another 2 inches for the window opening. I used adhesive tape to attach the vellum to either side of the card, re-assembling the A2 card. I added a thin strip of red cardstock to give the window a finished look. You may also want to add a second layer of cardstock to the inside of the card (the top part) to make it a bit sturdier.