不错 is bu2cuo4. The tone of 不 changes according to the tone of the verb it modifies, and can simply be stated as "it's fourth except before other fourth tones in which case it's second tone". Therefore: In front of a first tone, it's fourth; in front of second tone, it's fourth; in front third tone it's fourth; and in front of fourth tone it's second.
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March 9, 2014 at 3:11 PM不错 is bu2cuo4. The tone of 不 changes according to the tone of the verb it modifies, and can simply be stated as "it's fourth except before other fourth tones in which case it's second tone". Therefore: In front of a first tone, it's fourth; in front of second tone, it's fourth; in front third tone it's fourth; and in front of fourth tone it's second.