I assume the mould you purchased is one of these 3'x3' plastic frames with shapes like rocks or something. Keep in mind that each time you mix a new bag of concrete mix, you are likely to use slightly more or less water, no mater how careful you are. This means that each pour you do will be a slightly different color. The more water you add, the darker it will be, and the weaker the concrete will be. The most important thing to do in this project is to create a solid and level footing, and consider using 4x4 wire mesh to tie all of your pours together. As far as coloring your concrete, there are many concrete additives which can be purchased, but not usually at home depot. Go to a concrete supply company like Lone Star, or whatever company drives "cement-trucks" around in your area. Tell them you would like to purchase some concrete colorant for your backyard project, and they should be able to help you.
Make sure you use as close to the same amount of water for each sack of concrete as possible. Only add enough water to make the mix workable, but no more than that. the mix should keep its shape if you make a mound out of it. (ie stiff, like mashed potatoes!)
have fun! it takes a while to do this. You might consider buying more forms so you don't have to wait forever while each pour dries enough to pull the form.