There are many paints you can use in an altered book. I like the Golden's fluid acrylics. They are absolutely lucious. Lots of pigment, transparent, and flow beautifully. They can even be thinned down with extenders to be even more transparent. They mix beautifully as well.
Craft paints work well. This is the cheapest way to add color. It's thicker, so pages will have a different feel. Many are opaque. Metallics are available.
Rub on metallic are useful.
Watercolor can be used, but it warps the paper pretty badly. You will have to really glue down the paper to the next page to flatten it when it is dry. And maybe have to press it down. But there are times that is gives just the effect you want. Inks are the same. Used sparingly, they don't affect the paper much, but if used as a wash or on a larger area, it will warp the paper.
Fabric paints can be used.
Spray paints can be used, as well as mouth atomizers with your paints and inks.
To apply any paint you can use your choice of tool. Sponge brushes, paint brushes, paper towels, credit cards, spatulas, tooth brushes for spattering, etc.
Other ways to add color are crayons of all kinds, pastels, inks, collage with colorful papers or fabrics.