updating form values without submit

This seems very simple, but I'm not sure how to do it. Lets say that I
have
a form with two select boxes, call them Colours (red, orange, yellow)
and Items (contents depend on colour selected). If the colour is
changed from orange to yellow, I want the second select box to change
it's contents from "Oranges, etc" to "Bananas, etc" without the submit
button being pressed. How do I do this?

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