Do secondary container or workplace labels need to be printed in color?

According to OSHA, if the employer chooses to use the pictograms that appear in Appendix C of the HazCom Standard on their workplace (or in-plant) labels — also known as secondary container labels — these pictograms may be produced with a black border, rather than a red border. (source)

NFPA labels should be in color. The shades of red, blue, and yellow are not regulated but should be contrasting colors. The hazard ratings may have colored backgrounds with contrasting colored numerals or colored numerals with a white background. (source)