Here's a little secret just between us girls, if you group several objects, apply a fill to the group, Xara treats the group as one object. Better still, when you Ungroup the objects, the fill remains unchanged. (You cannot do this in CorelDRAW). Armed with this secret, group the two panels that make up the left side of the cube, then group the right two panels that make up the right side. Select the top of the cube, then select the Fill Tool and pick Fractal Clouds from the Fill Type drop-down list. The blue color is RGB 0, 153, 255. The green color is RGB 102, 153, 0. Click on the fill with the right mouse button. Change the Fractal Graininess setting on the Infobar to 3 (if it is not already set to 3).
Copy the fill to the clipboard (Ctrl-C), select the left side and Paste Attributes (Shift -- Ctrl A). Adjust the fill directional arrows as shown. Select the right side, Paste Attributes again and once more, adjust the directional fill arrows as shown.
Note: I've moved the panels apart slightly to enable you to see all three sides clearly.