Change color of a button after initialisation of the button

[HappyUser]

How to change the color of a button after initialisation.
For example : Button Pattern_8(0, 0, 0, 0, false, "Pattern_8", off_clrs, on_clrs, MC_DATUM); //TR_DATUM);
Pattern_8.off.bg=BLUE;
But the instruction gives a compilation error. Any idea? Thanks