The Orange Sulphur and Clouded Sulphur (Colias philodice) are often difficult to distinguish.  But the Orange Sulphur generally has some orange in the wing color, while Clouded is purely yellow.  Here in the Chicago area, at least for the past few years, it seems that Clouded Sulphurs are more common in the early summer and Orange Sulphurs more prevalent in late summer. 

White females of Orange and Clouded Sulphur are probably indistinguishable in the field.  But it was mostly Orange Sulphurs flying; my guess is that this was probably an Orange Sulphur.  The upperside, which was visible only during flight since they sit with wings closed, was very white. 

Caught by a Crab Spider that sits in flowers and waits for its prey, this Orange Sulphur is upside down with wings folded forward, and one is looking at the  upperside of the forewing.  Note the spots in the black border.  A male Sulphur has a narrower black border without the spots.