Ideal hair color for FUE?

There is nor ideal hair color for FUE unless a robot is harvesting your hair.  Robot cannot see white hair very well.

The
ARTAS robot is able to perform on dark hair, blonde hair, gray hair, very curly
hair, and ethnic hair. With ethnic hair we use a 1.0mm punch instead of a 0.9mm
punch. People who have white or light blonde hair need to have their hair dyed
before (or we can do it in office) the Robotic FUE procedure in order for the
robot to best visualize the hair follicles

it is wrong that you'll never have a youthful dense head of hair with fue. I can prove that with hundreds of my patient's photos. Dark , curly , or wavy hair gets better results after HT.But tricky point is dense packing FUE. During the placement of follicular units there will be a mixture of single, double and a few triple grafts, which help to create the soft look of a natural hairline. For the first few lines always go for single grafts, later doubles Triples and others.So that a smooth transition will be there giving undetectable hair transplants.

The optimal combination for the fullest appearance is typically a darker scalp and lighter hair, where the contrast between the scalp and the hair is minimized.  In general, lighter or gray hair tends to provide a fuller look.

That will cause the grafts to come out as the crusts are firmly attached to the grafts and when you pull or pick on it, you will see the graft come out

The color of hair in general does matter as far as giving the illusion of density, however for harvesting the grafts it doesnt matter. 

Hair color doesn't really play any part in the final result of the hair transplant. If hair is coarse and wavy it will take up more space and therefore more density compared to very fine and straight hair. However, darker hair can also create the illusion of density. For example, a person with black hair will look like they have more hair compared to a person with white hair even though they have the same amount of hair. .