The Sulphurs are infrequent visitors to my garden.  When they do visit, they stay a long time sipping nectar.  The males of each species have a solid black border on the outer margins of their forewings above.  The females have a spotted black border.  Both male and female have a black spot on each forewing.  The Common Sulphur is yellow above, and can be yellow or greenish-yellow below, depending on the time of the year.  The Orange Sulphur is orangey above, with a distinct orange spot on each fore wing.  It is usually brighter below, and can be golden-yellow, yellow, or greenish-yellow below.  Some of the female Sulphurs have an albino form, as shown on the right above.  The Sulphurs are hard to identify without seeing them from above, and they rarely rest with their wings open.  The albinos are even harder to identify.  The pictures on the left are a male (top) and a female (bottom), and are probably Orange Sulphurs.  The picture on the bottom right is a female, and probably a Common Sulphur.