gra·date 
(grādāt′)
(grādāt′)v. gra·dat·ed, gra·dat·ing, gra·dates
v.intr.
To pass imperceptibly from one degree, shade, or tone to another.
v.tr.
1. To cause to pass imperceptibly from one degree, shade, or tone to another: gradated the light in the painting's background.
2. To arrange in or according to grades.
[Back-formation from GRADATION.]