Using the Ellipse Tool, drag a circle the same dimensions as the back of the cube (160 pixels).

Change the fill color to RGB 204, 0, 255. Drag and flop duplicates the depth of the four sides (with out using the Ctrl key) as shown. You will have four ellipses when you are done.

Select the center circle, then select the Transparency Tool.

From the drop down list of Transparency Shapes, select Circular. From the drop down list of Transparency Types, select Bleach.

Select one of the ellipses and repeat the last step except select  Elliptical from the drop down list.

Repeat adding Elliptical Transparency to the other three sides.