The Clouded Sulphur and Orange Sulphur (Colias eurytheme) are often difficult to distinguish.  But the Orange Sulphur generally has some orange in the wing color, while Clouded is purely yellow, often a bright lemon-yellow. 

Some Sulphur females are white.  This is probably Clouded Sulphur, since I only saw those flying when I took this photo, but the white female for the Orange Sulphur is very difficult to distinguish from the white Clouded female. 

The upperside black border of Sulphur females (both the white and yellow forms) contains spots, while the male border is solid.  (See the Orange Sulphur.)  Although the right side is a bit tattered, one can see the spots in the border.