I love playing with different color combinations. Usually I just go about it on an intuative bases. I will play with my primary colors ( only order primary colors, plus white, black & ecru) until I get something that feels right.
I need to take a step back and talk about my color creating sessions. Often I will do my color creating when I am out of creative ideas for project, after I have had an intense bout of creating (usually getting ready for a show), or when I need to spark my inspiration fires. I find creating colors is very relaxing, almost a meditation for me. I get into another zone and just let it flow.
My color mixing is pretty organic. I will pick my primary color and start by adding white; just a little bit of white, or maybe a little ecru. I get my base color and then start playing with the saturation, the intensity and the hue. I am doing this mixing by feel, not by recipe. I do know artists that do follow recipes. I know artists that have created pretty large "recipe books" being very precise with the amount of each color added. They create little tiles for each of these color variations. In this way they can go back to create that exact color again. I definitely see the benefit in that, especially when I start working on a large piece that might even involve into an even larger piece.
Every one finds their own way to mix and work with color combinations. Color Scheme Designer is one of my favorite web sites for playing with color combinations.
Turquoise was a challenging color for me to create; trying to find the right balance between yellow and blue. Along the way to finding that true turquoise, I discovered other colors and learned how the additions of white, black and ecru influenced the hue.
Here are some links about the variations of color.