Have you ever used a marker, crayon, or oil pastels and had an unexpected color/mark appear? These materials can often "pick up" color from use, misuse, and abuse. I always warn my student not to color marker over top of marker. This can cause a transfer of color onto the tip of the marker that may have unappealing results for whoever uses that color next. Crayons and oil pastels can easily pick up the "crumbs" of other colors while in use or storage. Just being together in a box is enough to cause a transfer of colors on to each other. If you are not careful, these seemingly harmless acts can affect a work of art.
Whoever walks with the wise becomes wise, but the companion of fools will suffer harm. Proverbs 13:30
Do not be deceived: “Bad company ruins good morals.” 1 Corinthians 15:33