Hi, I would select the wanted color by opening photoshop, in reduced size, having the website showing behind or next to it. Get the Eyedropper, ( shortcut: I as I dropper) and click and drag on the image, until you reach an area on the webpage with the wanted color.
Now swich to the paint bucket tool (an alternate to the Gradient tool, So hit SHIFT+G to get the paint bucket tool, or click on hold on the gradient tool, until you see the alternate tools.
Now click on the areas to fill them.
If you also want to fill the bands, the best is maybe to first create a new layer 9the [+] icon in the Layers panel, and check the all layers in the paint bucket tool's option bar. this way you can change the blending mode to darken to retain the shadows. You can also do that if you change the Mode: from Normal to Darken for instance.