You should pick soft triquetrous pencils for a small kid. It is sufficient to have a set of six basic colors. When coloring, try pronouncing the name of each color, this helps the baby to remember them.
Images for older children may already contain small parts, complex narrative compositions with greater color palette. The outline is more subtle. These can be supplemented with coloring tasks. For example, to write the missing word, or paste a sticker, finish drawing, connect the dots.