To change the background color of MBProgressHUD comment this line and add this in MBProgressHUD.m in drawRect method:
//CGContextSetGrayFillColor(context, 0.0f, self.opacity);
CGFloat fillColor[4] = {0.0,0.0,1.0,1.0};
CGContextSetFillColor(context, fillColor);
This code changes HUD color to blue.