Keyboard shortcut to change font color in Libreoffice to a specific color

Muru's suggestion is close, but use Character styles, not Paragraph styles.

Press F11 and click the icon for Character styles.
Right-click Default Style and select New... Change the font color to red. In the Organizer, name the font color Red, then press OK.
Tools -> Customize, select the Keyboard tab. Find Alt+R and select it.
Under Categories, choose Styles / Character. Under Function, select Red. Then click the Modify button.
The Default Style Character Style is already black, so assign it to Alt+B. Then press OK.
Finally, select words and press Alt+R.

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