We changed hair color in a past post using the 'Brush Tool' and painted a red color over the hair for selection. Today, we will change the color of a shirt using the same idea but with a different tool.
(two more renderings of his shirt here)
Open your image (we'll use this one).
Right-click the 'Lasso Tool' and then choose the 'Polygonal Lasso Tool'.
The tool works by clicking once where you want to select and then moving the mouse a particular distance and direction away to form a line. Choose where you want to end the selection and then click the mouse button again. This allows straight selections to be done on fine edges. When you have clicked around the entire shirt and need to select no more, double click at your starting point (which should be where you end your selection if you are correctly moving around the shirt).
As you're selecting (since I'm sure you'll be zoomed in quite far) you can pan around the image to reveal more room by holding down spacebar and dragging the screen in any direction). Just ensure you don't double-click while you aren't holding down spacebar (else you'll end your selection right there).
Just try it out, it's easier than it sounds.
After you've selected the entire area, navigate to Layer > New Fill Layer > Solid Color...
Use these settings for the options:
Choose any color and select OK: