For purposes of controlling transparency of paints, adding gels and/or mediums offers a useful tool. Nearly all gels and paste are effective for this purpose, with the exception of those that are opaque (the Pumices and Molding Pastes).
The Gloss Gels are most effective, especially when highly transparent glazes are desired, and the glazes are to be applied thickly (greater than 1/8 inch wet film thickness).
The Matte and Semi-Gloss products will increase the translucency of the paint, but will not yield genuinely transparent glazes. This is the result of the matting agents in such products, detracting from the overall transparency.
The milky acrylic emulsion eventually dries translucent, and this must be kept in mind when determining the amount of color to add to gels and pastes.
I use Golden products exclusively, as I've found them to provide the best results. Matte gel medium makes a great glue for paper and also small embellishments. Use an old credit card as a spreader. If you spread the gel on both sides of the paper, you will prevent wrinkling as it dries. What causes the unsightly buckling and wrinkling is due to paper shrinkage. If you spread it on both surfaces, they shrink the same amount as they dry. (Be sure that the paper is laying flat to begin with. )