What binder (like egg tempera, milk paint, gum arabic, etc) to make natural pigment hand prints?

For one my kids' projects, they'll be making hand prints to form a tree, and then writing what they like about the earth next to them. I suggested using natural pigments, since they're even closer to the earth. But, what kind of binder would work? I've done milk paint, but that would crack off of the big piece of paper. I've made a watercolor with gum arabic, but that seems too think for hand prints.

Would a gauche work? How do I even make a gauche? Maybe instead I should try for an Egg Tempura? But that time of the year, my chickens and ducks should be making more eggs than I can use, so I might be able to make enough egg tempura for 30 hands?

Maybe there's a better binder that I don't know about? Or a mix of binders? I've got lots of raw pigments--I just don't know what to put them in!

I want to try mayonnaise paint

Although gouache looks pretty easy to make compared to watercolour https://permies.com/t/234982/art/gouache-paint-natural-materials

Oh, an article on making egg tempera paint

What a pretty chicken!

How to Make Tempera Paint with Eggs
Use pure artist’s pigment in powdered form and without fillers. These can be expensive, depending on where you buy them. Tinúviel orders from Kremer Pigments and found they have good quality and fair pricing.

“Expense ranges based on the source of the pigment, whether colored earths or rare pigments. The most expensive colors tend to be the reds, blues, and yellows. They don’t have to be expensive, but if you want some really nice, bright colors, the costs do rise a bit. If you look at old paintings, you can understand why blue was reserved for only Virgin Mary or very specific highlighted features in a painting. The cost of materials. Fortunately, a little goes a long way.”

Mix pigments with egg yolk in fairly equal amounts. Do this a little at a time; if the paint dries, you can’t reconstitute it. Only mix up as much paint as you will use before taking a break. Then add enough water to reach the right consistency.

But won’t the egg yolk turn the colors yellow? Online tutorials about how to make tempera paint say no. Tinúviel also assures that the yellow color fades as paint dries. White paint remains white. The water also doesn’t diminish the amount of color deposited. It just makes paint glide on easier, to allow different textures and effects.

Build up layers as you paint because each layer affects the transparency and opaque nature of the paint above it. Be sure each layer fully dries before you apply another to avoid paint that later comes off the panel.

After each painting session, wash brushes fully. Egg yolk dries hard and is difficult to remove; egg tempera can ruin the most expensive brushes.

Completed paintings may be perfect with a matte finish, or you may want a bit more luster. With a soft brush, apply a final coat of Damar varnish or shellac over your artwork to give it a bit more shine. Tinúviel prefers hers matte.

(special thanks to the person who pointed me to the article.)

I recently learned that Rembrandt (and others of that time) used tempera paint layers with their oils.  They would do a layer of tempera, then a thin layer of oil, then more tempera.  The oil being mildly transparent and applied thinly allowed the texture of the white tempera to show through and make parts of the painting glow.  

Unfortunately, the instructions in that book included a lot of modern-day solvents to make the tempera.  But the more I learn about painting, the more I want to try tempera.  Pre-Renaissance Europe, it was one of the main paints and many of these still look stunning today.