Need help with matrix in Graphic. C#

I found this code online that can help me fit my image in my panel as best as possible.

double largestRatio = Math.Max((double)image.Width / this.ImageDrwPanel.Width, (double)image.Height / this.ImageDrwPanel.Height);
            float posX = (float)(this.ImageDrwPanel.Width * largestRatio / 2 - image.Width / 2);
            float posY = (float)(this.ImageDrwPanel.Height * largestRatio / 2 - image.Height / 2);
            mx = new Matrix(1.0f / (float)largestRatio, 0, 0, 1.0f / (float)largestRatio, 0, 0);
            mx.Translate(posX, posY);
            this.statusBarPanelZoom.Text = "Best Fit";
            memGraphic.Transform = mx;

This code work perfectly and it fit the image in my panel as best as possible,  but I wanted to expand on this code and fill in the rest of the panel with a different color.  Such as red or yellow.  Can some experts tell me how can I fill in the rest of the background with another color?  I've been trying to generate another matrix to draw in my memGraphic, but I can't seem to make it working properly.  

PS

I overrided the OnPaintBackground method because I don't like the way it render the background and I wanted to generate the background manually.