You need to create different cellstyles with different background colors based on Default cell style. You must to name those cellstyles as color names, When you select (or type in) a color name into a cell, the applied "Conditional format" option will "compute" the style name from the cell content, and will apply it on cell.
Tibor Kovacs; AOO3.4.1, LO3.6.4, LO4.0.3Portable on WindowsXPprof.SP3
You can select all the cells you want to apply conditional formatting and write the formula STYLE(Cell wich have focus). Or format one cell and copy to others with the paintbrush. Or copy/paste special > Formats