introduction to "צביעה באוכלים" (coloring foods)
Is There a Problem with Spreading Foods (e.g., Peanut Butter, Cheese, Hummus, etc.) onto Challah, Matzah, etc. on Shabbos?;
the bottom line on staining a cloth or a towel by wiping one's hands on it; wiping one's hands on the same color towel;
More Details About the Floor of a Sukkah. Sweeping a Carpet;