Sounds like what you need is to learn layering & glazing.
In your case, find three shades of purple. A highlight, a mid-tone, and a shade. In an artists' pallet (preferably ceramic, as they're easier to clean) designate 5 cups. In the middle cup place 3 drops of your mid-tone. In the cups adjacent to the mid-tone, place two drops. In two outside cups, place a single drop of your mid-tone. Designate one outside cup for your shade and the other for your highlight. Place two drops of shade with the one drop of mid-tone. Then place one drop of shade in the cup with two drops of mid-tone. Repeat the process for your highlight and then mix the colors with an old brush.
I know this may sound a bit confusing, but the point is to create a highlight and shade that share the mid-tone so they look natural on the model. The two cups adjacent to your mid-tone should appear to be lighter (or darker) than the mid-tone but not as light (or dark) as your highlight (or shade). To make things clearer, here's a bit of a diagram to show they should appear on your pallette. S=1 drop of shade M=1 drop of mid-tone H=1 drop of highlight.
SSM SMM
MMM HMM HHM
Mixing the colors is only half the battle, as the paints have to be thinned and applied in layers. This way you allow your base coat to show through which creates highlights and shadows that appear natural. There are many different techniques for thinning paint and even if you use the same methods every time, adaptations will be required based upon the paint's brand, color, and batch. As a result, patience and test models are absolutely necessary. What I describe below is an oversimplification of a single technique and should not be considered gospel by any means. Experiment, ask others for their opinions, and do additional research. For the record, I use Vallejo Game Color almost exclusively.
GW or other paints will be different.
I always have a batch of 1 part Flow-Aid mixed with 10 parts water on hand. Flow-Aid is a surfactant, meaning it reduces the surface tension of water. This is generally mixed with the paint at a 1:1 ratio to start. I then add a few drops of Vallejo Airbrush Thinner, some glaze medium, and a single drop of Slow-Dry. The airbrush thinner is about the thickness of water but allows the pigments to bind so they do not break apart on the surface of the model. This can happen if the paint is too thin and the glaze will look like crap. Glaze medium also contains acrylic binders and aids in the transparency of the paint. Adjust your ratios as necessary and keep a written log of what you do so that it can be duplicated in the future (or not depending upon the outcome). In any case, when the paint is pulled up the side of the pallet, it should flow like milk in a glass. Grab some milk, swirl it in a glass and see how it runs down to get an idea.
Slow-Dry is used because my preferred technique is called "wet layering" wherein you apply a layer of paint, clean the brush, and then blend the edges of the layer. Slow-Dry slows the drying time of the paint and makes this possible. However, it should be used sparingly as even a little too much can cause strange effects.
Other techniques include layering (applying thin layers of paint on the model in a form akin to a contour map) and wet blending (placing a mid-tone and highlight [or shade] next to each other on the model and blending them together on the model). Research them all and see what works for you. In virtually all cases, the point is to apply thin layers repeatedly until you achieve the desired effect. If after one application of paint on the model you can barely see the results, you're doing it right.
Many resources are available to learn how to paint. Websites, books, DVDs, etc. I'm still on the learning curve but navigating it at a steady pace. I can honestly say that Google has taught and will continue to teach me how to paint miniatures. Treat painting like playing an instrument. If you really want to have great results, then be willing to do the research and practice often.
I hope this helps get you on the right track
