We should be able to choose the color of the button on a button grid by entering either RGB or HEX codes. We can only see the RBG codes only once the color is set on the button properties window.

With the current setup, it is difficult to choose a color for multiple buttons. If you are adding a new button to an existing grid and don't really know what is the color of the current buttons, we have to waste a lot of time doing trial and error methods until we get the exact match.