It looks like you are doing what is described here: http://docs.gimp.org/en/gimp-tool-color-balance.html. I don't know how GIMP works internally but it looks like you are just adjusting the values of each channel in an image. The following might work for you:
image.Evaluate(Channels.Red, EvaluateOperator.Add, 50); image.Evaluate(Channels.Green, EvaluateOperator.Substract, 20);