Electrical – connect the green wire from the dishwasher

The instructions for the fixture are only correct for a metal box. If a metal box was used, the box itself would (should) be grounded. The bracket that holds the light would then be connected to the box, which would make the bracket grounded. Finally the ground wire from the fixture would attach to the bracket, grounding the fixture.

In the case of a plastic box, the box is not grounded. All ground connections must be made by connecting the ground wires together. So in your situation you are correct, you'll connect the ground from the supply to the ground from the fixture using a twist-on connector (or other approved connection).

Where are you located and how old is the house?

In the U.S. there are some standards for household wire color coding: http://www.angieslist.com/articles/what-do-electrical-wire-color-codes-mean.htm

If you own a multimeter, you can measure the voltage on the yellow and blue wires. Determine what the voltage is and if it's correct for the dishwasher. You will also be able to tell which wire is hot and which is neutral.

The black on the dishwasher is your hot wire, and the white is the neutral.

If you are not able to, or not comfortable testing the wires, please hire a professional to do it for you. Household electrical can be dangerous and must be done correctly.