I wanted to have a div appear like a 3D sphere and transition through colors. I discovered that gradient background colors don't transition (yet). I placed a radial gradient background in front of the element (using z-index) with a transitioning solid background.
/* overlay */
z-index : 1;
background : radial-gradient( ellipse at 25% 25%, rgba( 255, 255, 255, 0 ) 0%, rgba( 0, 0, 0, 1 ) 100% );
then the div.ball
underneath:
transition : all 1s cubic-bezier(0.25, 0.46, 0.45, 0.94);
then changed the background color of the div.ball
and voila!