Combining blue and yellow creates green. However, the shade of green created is altered depending on the ratio of yellow to blue.
About.com suggests starting with yellow and adding small amounts of blue until you get the desired shade of green. This is because it takes less of the darker color to change the mixture. It would take adding much more yellow to blue to get the same shade of green.
The shades of blue and yellow are another determining factor. When mixing colored lights instead of paint, it is important to note that green, along with red and blue, is one of the three primary colors.