I'd like to share with you the images of the cactus stamp as it was watercolored in. Hope it's helpful.
First stamp the image on a smooth toothed watercolor paper. If the watercolor paper has too much texture to it you will not get a good impression from your rubber stamp.
I used Stazon Black. Any solvent based ink will do. Other inks will smear when you start to watercolor in the image.
I use different sizes of sable art brushes when I watercolor. This way I have more control over the amount of water that is on the bristles.
First I wet down the paper in the areas that I want to apply my ink (or watercolor paint). I like to do this because it makes the ink spread out more along the surface of the paper.
First layers of dye ink 
are SU! Mellow Moss and Rose Royal. On the flower petals I applied additional layers of Rose to darken them.
Second photo I started to add some more depth to the image but adding another layer of Mellow moss. I now added Summer Sun (Yellow) to the flower. I also added some green to the petals to create some shading.
Third image. I'm now adding more layers. The flowers have Really Rust and More mustard added to the center. You can see that I've added some yellow to the greens on the cactus body. This again gives the plant more depth.
I like to introduce more than one color to images that I 
want to look more realistic. I work in layers. Layers are truly important when you watercolor. But be careful. You don't want your image to get muddy.
How does it get muddy looking? By adding layers that are too heavy. Light layers on top of light layers create beautiful depths. It's all about practicing. The more you do it, the better you get. We all make the same mistakes at first. Trust me. I had many muddy watercolored paintings when I first began.
Fourth image. I now am adding Always Artichoke and Ruby red to the image. I wanted to deepen the color on some areas on the body of the cactus and the flower petals.
I think often the hardest part of watercoloring is trying to visualize where the shadows will be. Where the highlights are. Remember, when watercoloring you go from light to dark. Layers are added to the areas that you want the shadows to fall and less, if no layers are added to your lightest colored areas.
Visualize the sun hitting the cactus out in the desert. Where would the rays be hitting it? Where would the shadows be cast?
Fifth image. I used a clean wet brush and went over the Always Artichoke layer. This helps to smooth out the colors.
Remember, I'm working rather fast. The paper is not completely dry while I'm doing all of this watercoloring. So it is rather easy to still move the pigment along the surface of the paper.
If you wait until the layers dry, you will get lines instead of a wet look on the surface of the paper, as I have here.
Tip: Don't soak your paper down with too much water. It just needs to be a bit damp when you start to color. If the paper is too wet then you get layers that smear together and get muddy looking when you start to add colors on top of each other. If the paper is too dry then you get layers that are too defined. You get what is called "dry brushing". Your paint brush needs to be wet but not soaking wet. Since we are generally coloring in smaller detailed images, you need to learn how to control how the pigment goes onto the paper. Unfortunately, there's nothing that can help but practice!
Here's the finished card http://hopartstudios.typepad.com/hop_art_studios/2007/10/cactus-dreams.html
white craft ink pad I proceeded to ink over the entire area of the image. Craft ink takes a bit longer to dry, which is a good thing. I took a paper towel and wiped over the surface. The white ink adheres to the expossed paper but not to the raised embossed image. So what you get are darker areas beneath the embossed areas and lighter areas in the areas not embossed.
Sometimes simple can be just oh so perfect.
