I suspect she may have a hunch, as there have been many other clues of his philandering over the years. Telling her brings it out in the open, forcing her to confront her husband, rather than ignoring it in the hope that it will burn itself out. I've had to return to work to deal with pressing business. I know the old rules said black for a year, but I am naturally pale and look especially washed out in black. Rules for a widow's garb have changed.
Many people wear whatever they want to right away.
Old-time etiquette mandated black attire for a year, then a gradual evolution to more somber colors, finally changing into the pastels. I espouse darker clothing for a few weeks, though it needn't be black. Afterwards return to your normal clothing. Normal wear is perfectly appropriate for your daughters. When business associates ask about your husband, tell them the facts and accept their sympathy graciously, but try not to dwell on it.