The performer shows a square tube. The tube can be folded flat and is seen to be just four pieces of cardboard held together with green tape. The audience is allowed to look through the tube from both sides. The tube is decorated on the outside and has colored stripes on the inside. We will presume these stripes are red. A silk of contrasting color, say blue, is placed inside the tube. The tube is smashed flat, a magic pass is made over the tube. When the tube is opened again the silk has changed from blue to red and the stripes inside the tube have changed from red to blue. The tube is once again shown to be empty. Complete with 2 silks and detailed instructions.

TUBE IS 4.25" SQUARE BY 8.5" TALL

Demo:

https://www.youtube.com/watch?v=9Gxaj0_YxfQ