Next, I select each panel individually using the lasso tool, and begin filling each of them with a separate background color.

Some people use option delete to fill, areas, but I prefer the Bucket tool: even though I have both white and blue-gray areas selected, when I hit the blue area with the Bucket Tool, only the areas with exactly the same color as where I click fill with the new color. You can also use the Magic Wand tool with Command-Shift to isolate the area you want to fill, but that just becomes an extra step you have to make every time you want to fill an area with color.