Photo tools are a set of useful tools that can be used for manipulating an image.
This tool 
is used to choose a color exactly as present in any part of your image.
The color you pick will be reflected in the color wheel. Left-click to choose the primary color and right-click to choose a secondary color.
The Clone Stamp tool 
gives you the ability to take a region of a photo and copy those pixels and paste them onto another region.
This will be your go-to tool when fixing blemishes on a photo. You can use this tool by selecting the area you want to copy using Ctrl + right-click. Then, move your mouse to the area you want to fix. Left-click and move your mouse over it. Your two areas will stay equidistant from each other as you move your mouse. You can reset the selection by selecting a new area you wish to clone by using Ctrl + right-click again.
Note
If you want to remove a blemish from someone's face in a photograph, it's best to use the Zoom tool and get in as close as you can, then use a very narrow brush width. If you choose an area that is colored the same as the area the blemish is, you can easily give someone a clear completion with a few clicks.
The Recolor tool 
will replace one color with another.
Using this tool, you can change the color of someone's shirt or the color of a car. It takes a little finesse to use it smoothly, but once you do, it can create some really interesting effects.
The first thing you want to do is select the color to which you are going to change your selection. You can do this using the Color Picker in the color wheel or by selecting the color from another part of your image by pressing the Ctrl key and moving the cursor to the area you want to select. Doing this will make the Recolor tool act like the Color Picker. You can choose your secondary color using Ctrl + right-click.
The secondary color in the Colors window is the one you want to replace. For instance, if you want to turn a blue sky red, hold down the Ctrl key and select the blue color of the sky. Select a secondary color in the Color Picker; use the Color Picker to choose the red color that you want to replace it with. Left-click and start painting the blue sky, and anything that is blue (the primary color) will be replaced with red (the secondary color). What's neat is that only the areas of the color you chose as your primary color will be painted. This can be adjusted with Tolerance. With this, Paint.NET will identify the edges that you are recoloring and the rest of your document won't be changed.