If there is a focus shift of the projected image with different colored light then the enlarger lens has chromatic aberration.
When using a grain focuser with an arial image, if the reticle is in focus on the retina then chromatic aberration of the human eye will be compensated.
That is not true. There was no measurable focus shift of the lens itself. I proved this by making prints with red, green and blue light after focusing visually with green light, using the grain focuser. None of these prints was out of focus. On the other hand, refocusing with red, green and blue light using the same grain focuser produced out-of-focus prints with red and blue light. You see, the eye is most sensitive and has its highest resolution in green light. The resolution is markedly less in blue and red. Both the image and the crosshair are fuzzier in red or blue light. The image of the crosshair cannot be in sharpest focus on the retina in red or blue light if it was set in green light because of the chromatic abberation of the eye. The combination of focus shift of the eye, which you can easily demonstrate to your self by measuring the nearest point of clear vision in different colors of light, and reduced resolution gives a wider spread in focus errors as well as a bias of the mean.
There is no point in arguing the theory when the theory does not explain the experimental results. The lens I used was a Rodagon apochromatic which is optimized for reg, green and blue.