First I coloured each circle within the composition by double clicking on the layer in the Layers Palette. This brings up the Layer Style palette. Select Color Overlay, then select the small coloured rectangle within the palette. From here, you can fiddle about to achieve the exact colour you wish.
Apply outer glow and drop shadows to the circles through a similar process, all the while playing around with different results until you’re satisfied with the illustration’s overall harmony.
Next move onto your other elements. Here, I added Outer Glow to the Mitosis, Sperm/Egg and Gummi morph elements. To increase the opacity, for example on the Sperm/Egg element here, place a duplicate layer over the original layer by rightclicking on the layer in the Layers Palette and choosing Duplicate Layer.
I wanted to add some stripes to liven up the grey background of my illustration. To do this, create a new layer (Cmd/Ctrl + N). Select the Rectangle Marquee tool, and click and drag to create the desired shape and filling with a desired colour (Edit > Fill). Duplicate the stripe across the background by holding Alt while clicking and dragging the original stripe.