Go to your paint program (I use MS Paint)... Get the RGB value of the color you want...
Go to OR, double click on the part you are changing color.
Open the "Appearance" tab, and uncheck the "Appearance ☑ Use default" box (in the lower section of the window).
Click on the "Color: □" box (default is typically black), and swatches of color appear.
Click on the "RGB" tab.
Enter your color's RGB values then click "OK"
Regrettably, OR doesn't keep a "pallet" for you to save custom colors with. So, you'll need to enter the values for each part.
I don't know how to get custom values for: Hue Saturation Value (HSV), Hue Saturation Light (HSL), Cyan, Magenta, Yellow, Black, Alpha (CMYK), and thus don't use those features. Though now I wonder if any one of those could be used to simulate chrome or some other metallic color accurately.
You can also create a small "decal" (aka Texture) of the color, then apply that like you would any other graphic in OR.