You can reduce the value of the red channel like this:
The 10 is probably not correct because I don't know what '10 points' means.
And you can find some help about writing a Parallel for loop here: https://msdn.microsoft.com/en-us/library/dd460713(v=vs.110).aspx
mgkImage.Evaluate(Channels.Red, EvaluateOperator.Subtract, 10);
And you can find some help about writing a Parallel for loop here: https://msdn.microsoft.com/en-us/library/dd460713(v=vs.110).aspx