Mh, then there is no obvious reason for this darkening. You could use your reference output and compare a white-tinted image (= implicit conversion to grayscales) of just a pure red, green and blue area to derive the weights they used, to make your output similar to it. That is, if you approve of the reference
"When one person suffers from a delusion, it is called insanity. When a million people suffer from a delusion, it is called religion." (Richard Dawkins)