Is the perceived color change at the same line as the old green? It is possible that the old green color could be affecting the new color. If this is the case then another coat is needed.
Another possibility is the angle of light that is hitting the wall. The color difference could be simply a trick of the lighting.
For future reference; For dramatic color changes using a primer, white or tinted, is a good idea. It will seal up and block the old color. Giving you a neutral canvas for the new paint color.