There are several ways to do it depending on how you define green. See my File Exchange for three color segmentation tutorials. One that does it in RGB color space, one that does it in HSV color space, and one that does it based on the Delta E color difference metric.
Change the tutorials to get green rather than whatever the color the demo gets. It's just a matter of changing some threshold values. You don't need to change anything with the Delta E method since it does it based on some area that you draw in the image rather than on some absolute color values.
Just as background information, to understand how the calculation in MATLAB works: when you load a figure in Matlab, an array will be shown in the workspace (i.e. 400x400x3) At each pixel, the colour of that layer is defined (in rgb). If you for example convert it to grayscale with the functionrgb2gray it will result in a 400x400 array since you only need one value at that pixel. (how black-white the pixel is).