Buzzing sound from junction box when dimmer switch is on
Yellow is indeed not a ground

Your house was wired up using conduit (rigid and flexible) instead of cable -- therefore, the electrician could use whatever wire colors he so pleased (with the only ones off limits being white, grey, and green) instead of being stuck with the NM-cable defaults.  Furthermore, the conduit itself is a grounding conductor, so there's no need for further grounding wires running inside it.

So, simply use a green or bare jumper to a grounding screw in the box (there are dedicated holes in the back for a standard 10-32 grounding screw) to ground your dishwasher.  And the yellow is likely a switched hot indeed -- the buzzing was the dimmer's way of complaining about the wiring.