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The color of a block equipped in the toolbar can be chosen prior to placing it. To do so, use the '[' and ']' keys to cycle forward and backward (respectively) through the preset options. The color of an already-existing block can be changed by clicking the Middle Mouse Button (MMB) on the block with the desired color selected.
The Color picker can streamline this process. It is accessed by pressing 'P' (default) while not seated at a cockpit. Using this tool, any of the preset options can be chosen or a custom color can be defined.
In addition, when highlighting a block of a certain color, holding shift and pressing 'P' will act like an eye-dropper tool and copy the color for the next block(s) you place.