The little red pins are used to hold balsa parts together while the glue dries. Or to hold parts in alignment. They can also be used in foam board construction. The vertical stab and rudder are pinned down to make it easier to apply the tape hinge. Hard to do just holding the parts together while applying the tape.
The color of the FliteTest water resistant foam board is very similar to the color of cardboard. Wouldn’t it be nice if someone made tape the color of cardboard for use on that foam board? Well, someone does! I found this product at Amazon but it is probably available elsewhere too. Just what I needed. I tried to cut the elevator bevel free hand on my Old Speedster and cut through the paper hinge. The repair looks good and the tape added very little weight.