Like many tools have the option of doing, you can try and use blend modes for applying the tool. This will determine how the tool 'reacts with' the light of the layers beneath it. Changing blending modes here causes you to add or subtract light each time that you use the gradient tool...
Welcome to a magic wand exercise/demonstration. Choose the magic Wand tool (w). Note in the options bar you can change the tolerance. What this means is the color range that it is going to select...
Now that you've got a selected area (the wall in this case); go to Image: Adjustments: Hue/Saturation. Click on the Colorize box. Now you can move the Hue sliders around to choose a value which you can preview in real-time to see your wall change color...
For this tutorial grab the lasso tool in the toolbar. Now make a general selection by dragging the lasso around your subject. Try to keep it as close as you can. You have to go all the way around the subject to meet endpoints to close off the selection...