There is a considerable degree of variation in shade of color between weeds of different species.

The shade of color of some plants tends to change appreciably over the course of their growing season.

When crops are weeded manually, overall size and leaf shape are taken into account in distinguishing crop plants from weeds.

Selection and genetic manipulation allow plants of virtually any species to be economically bred to have a distinctive shade of color without altering their other characteristics.

Farm laborers who are responsible for the manual weeding of crops carry out other agricultural duties at times in the growing season when extensive weeding is not necessary.