These are House Finches. The males usually have red heads, but like flamingos, their red color depends on their diet. Carotenoid pigments give them their color, but different food types have different carotenoids. If their diet is missing the carotenoid echinenone, they will be orange. If they are lacking echinenone and isocryptoxanthin, they will be yellow instead, so their color varies, depending on what they eat.