In addition to massive (rather than minor) orthographic differences, Bokmål and Nynorsk have very different grammars (some examples: the word "I" is "Jeg" in Bokmål and "Eg" in Nynorsk, the word "not" is "ikke" in Bokmål and "ikkje" in Nynorsk, even the word "Norway" is "Norge" in Bokmål and "Noreg" in Nynorsk.)
Well, that's a vocabulary difference, not a grammar difference. But I believe you that there are also some grammar differences :), although the examples you gave look pretty minor to me, like color/colour...