The transparancy/opacity (antonyms) denote how much of the image begind the layer in question can be seen through it. Glass, for example, is motly transparent, whereas a wall is opaque. A thicker, green, glass could be done (though not that well, you'd have to add reflections and stuff to make it look good) by making a layer on top of everything you want the glass to be in front of, painting it the green (or any color) you wanted, and setting it too an opacity of around 20% (this is done in your layer window, in the top right corner) |
