Nikon Ai-s cool color cast va Canon FD warm cast

[BradS]

You need to use the same film, the same exposure and two cameras side by side, with the shots taken at or near the same time.
The film needs to be slide film, and you need to have it developed by the same lab, preferably in the same run.
Try to do the test under a number of different lighting conditions. And I would aim for something like f/8 for a significant percentage of the tests, although it might be interesting to see if the colour differences diverge nearer maximum aperture.
That approach should help isolate the variables to just the difference in the lenses.

...or one could just photograph with what one has and not worry about minutia.

hmmm...is this another (good?) reason to use only one camera and one lens until one has a firm grasp of what one is capable of and what one wants and does not want, photographically speaking.

 

[MattKing]

or one could just photograph with what one has and not worry about minutia.

Yep
(and I appreciate the decision to use "minutia", rather than the more commonly encountered minutiae)

 

[George Mann]

The film needs to be slide film, and you need to have it developed by the same lab, preferably in the same run.

Let me add that it needs to be viewed directly with a good loupe and light source (sunlight works best).