As can be seen in this picture there is yellow tones all through the light wood areas. This is because it is translucent oil applied. In about six months that lighter wood will start to change to beige and tan tones as the wood ages. These are "living" and will change as they age. Otherwise you would need to sand to apply a finish.

If there is much color variation among the stools, is it possible to request one with less yellow/more white/gray when ordering?

Sorry not possible - every single unit is absolutely different in "color" as it is a translucent oil so you see the natural wood colors and grain through the oil. No two units will ever be alike in grain, wood color, or finish. That is what makes them unique as no two will ever be the same.