There are a lot of pre-made colors to use, which can be access as such
myInkCanvas.Background = Brushes.GhostWhite;
To adjust to your updated question
int r, g, b;
//set r,g,b to something
myInkCanvas.Background = new SolidColorBrush(Color.FromArgb(255, (byte)r, (byte)b, (byte)b));
And for fun, if you have a hex string
myInkCanvas.Background = (Brush) new System.Windows.Media.BrushConverter().ConvertFromString("#FFFFFF90");
With separate hex string
myInkCanvas.Background = new SolidColorBrush(Color.FromArgb(0xff, 0xff, 0x90));