The Tools, History, Layers and Colors windows are crucial to getting to getting anything done in Paint.net, but if you happen to close one, it's not entirely obvious how to open it again. This can leave you without access to the tool selection and the colour wheel. There are two ways to make any closed windows reappear. You can use keyboard shortcuts (press F5 for Tools, F6 for History, F7 for Layers and F8 for Colors). Alternatively, you can click the relevant icons at the top right of the Paint.net toolbar (see screenshot below).
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