For interpolation, averaging, etc. anything based on RGB is not going to give good results. For example, if you average pure red and pure blue, you should get something like pure purple. But if you do that in RGB you'll get #7f007f, a much darker color than either pure red or pure blue. If you do it with LAB, you'll get #ca0088, which is perceptually about as bright as either of the colors you started with.