The spelling “realize” is an accepted alternative in British English. It is even the recommended spelling according to The Oxford Style Manual.
You can however make an allowed spelling disallowed by MS Word, by adding it to an exclusion dictionary. The following worked in my environment where I have MS Word 2007: I opened the folder C:\Users\Jukka\AppData\Roaming\Microsoft\UProof\ (“Jukka” is my user name), and there I opened the file ExcludeDictionaryEN1409.lex in Notepad, after checking from the list Locale IDs Assigned by Microsoft that EN1409 is the Microsoft code for English - New Zealand. I entered the word “realize” (without quotes) in that file, which was initially empty, saved the file. Then I closed and restarted MS Word. Now that I set the text language to English (New Zealand) and type “I did not realize what the color is”, both “realize” and “color” are flagged as incorrect. Before the change, “realize” was accepted, as described in the question.
I learned this from the Office Natural Language Team Blog entry Can I remove a word from Office’s speller dictionary?, which was linked to from the page How to “remove” a word from Word's main spelling dictionary (which contains instructions for older versions of MS Word, too).
Due to the way spelling checks work for English in MS Word (basically simple dictionary lookup, instead of morpheme analysis), you need to add the grammatical forms and derivations of a word separately, so in this case, you would make the file contain
When I tested things with just “realize” in the file and right-clicked on “realize” marked as misspelled, MS Word showed a list of alternatives containing “realizes” and “realized” too, which was rather misleading. After adding the other forms, MS Word now shows “realise” only as the suggested correction for “realize”.