Draw a 3D Cube with Freehand tool...
(i have named the faces for help)

Fill Face 1 with C16 M95 Y0 K0
Face 2 with C24 M96 Y0 K0
Face 3 with C40 M100 Y0 K0 or Blue Purple
Then select all the three faces and right click No fill in the CMYK palette to get rid of outlines

Now draw three boxes over the faces and fill them with C76 M98 Y44 K54

Add transparency to the dark boxes....

draw one more box on face 1...fill it with C23 M96 Y24 K0
and then add transparency

Add a white stripe with freehand hand tool

Make the stripes transparent...this will make the Cube look glossy

Now with Bezier tool draw 2 objects on face1 and face2 (as shown in figure)...fill them with white color...and dont forget to remove outlines

Add transparency

Draw another stripe on the bottom of the cube....fill it with some dark color (C76 M98 Y44 K54)

Now add 50% transparency to the stripe...
Go to the transparency property bar...then type 50 in the transparency midpoint box...this will make the stripe 50% transparent

Draw two boxes and a white stripe with freehand tool...
fill the boxes with C24 M96 Y3 K0

add transparency... there is a small white slider between the transparency vector handles... drag this slider towards the white vector handle...

and done.....

change the colors and you can create many beautiful glossy cubes...