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The default colors for Usage and Generation can be changed from the Preferences menu. Select Settings from the top right corner of the screen. Next, select Preferences from the menu on the left side of the screen. Navigate down the page to "Color that represents energy usage (negative numbers)" and "Color that will represent energy generation (positive numbers)". Clicking the colored box containing color hex codes will bring up a color selection palette. The selected color(s) will replace the default red/green coloration on every register (including the shaded areas "Energy Used" and "Energy Generated") except those that have different colors/line styles specified on the Preferences page. 

You may use custom register styles to change the color and line style of a register. This can be done by clicking on Settings -> Preferences, and pressing "Add Custom Register". You are presented with the choices: 

Register   Which register should appear in the defined style
ColorColor for the chosen register
LineLine style and thickness
SignWhether to style the line for positive, negative, or both sign