ABSTRACT: Detection and resolution thresholds for blue and yellow targets against blue, green, and yellow backgrounds were measured while wearing yellow and blue filters. Yellow filters improved thresholds for yellow targets against blue backgrounds. Their effectiveness decreased (i) as the wavelength of :he background increased, (ii) as the size of the target decreased, and (iii) with the age of the observer. The blue fillers were generally ineffective. The results are for the most part explained on the basis of changes in target contrast brought about by the filters. The effectiveness of yellow facemasks for divers under certain conditions is discussed.