What color of toe kick (under kitchen cabinets) should be used??

mine matches my cabinets but you could also match the wall colors, the floor colors or use a metal that matches to other finishes. Black or grey can be like an anchoring color if you can't decide. since it is your house, you should do what makes you feel happiest.

ideally the same color as your cabinets.

What color are your cabinets ?

What color is your kitchen?

Black is always nice as it is dark under there and black makes it even darker.

There is no true rule.

When I worked for a flooring company, there was always an option for the home owner to go with or without coving in regards to the linoleum or ceramic tile.

If flooring will not have coving, then match the cupboard.

Good cupboards have matching toekick.

Research toe kick drawers...cool storage

the same color as the cabinet

It depends on the cabinet's color?