Translation:I want the red boots, not those black ones.
Because to use different forms of "gli" it's the word that immediately follows it that matters, which isn't always the one it refers to. So in this example, though you refer to "quegli stivali" you really say "quelli neri" because (those black ones) is what's being translated. And "neri" doesn't begin with a Z or an S+ consonant. Hope you understand now :)
I thought it was impolite in a store to use voglio; instead one should use vorrei. Why doesn't that apply here?
When I asked our exchange student she replied that it is in the tone and it can be taken as rude to use voglio, especially around people you don't know that well...
Then she told me of an old Italian proverb: l erba voglio cresce solo nel giardino del re! I read the story and got the gist... http://chimera.roma1.infn.it/GIORGIO/favole/erba_voglio.html