DENVER—A new law going into effect in Colorado Aug. 3 that allows workers to sue employers over discrimination based on sexual orientation.

The law is among more than 100 that went into effect that day. Others allow gay couples to adopt children, allow pharmacists to redistribute cancer medication and impose bigger fines on big-rig truckers who don’t chain up in bad weather.

Beth MacDonald, an employment attorney, said she is telling employers to document performance problems if they plan to fire a worker after the law barring sexual discrimination goes into effect.

Mindy Barton, legal director of the Colorado Legal Initiatives Project, a gay rights support group, said about 20 percent of the 350 calls a year the group gets seeking legal advice involve discrimination over sexual orientation.

The Colorado Division of Civil Rights estimated it would receive 80 complaints a year, with about 10 providing enough evidence to pursue.

Religious organizations are exempt unless they are supported by taxes.

MacDonald said Colorado is the 20th state to ban workplace discrimination based on sexual orientation and the 12th to ban it on the basis of gender identity.