On the Android version it seems to me that the tone marker that is accepted as the correct answer does not change even in situations where you’d verbally use a different tone. For example 不是 would be written as bù shì even though it would be pronounced as bú shì. And that seems to be the general trend when writing pinyin.
With that said, I suppose it’s probably a mistake then that the accepted tone markers for 不错 are bú cuò. Sure that’s how you’d pronounce it but I dont believe that’s the proper pinyin.
Skritter is meant to always apply tone changes to 不. If it doesn’t, there’s something wrong and we’d appreciate if users help us out by reporting it! In the case of 不錯, though, it seems that everything is as it should, which is true for 不是 as well. When I bring these words up on my account, they are both written with bú, which is how it’s pronounced. I already linked to this blog post in the reply to your other question, but I’ll post it here to in case someone sees this but not the other one:
Well the accepted pinyin for 可口可乐 is Kěkǒukělè when in reality you’d only pronounce the final third tone as a third tone and the first two would be second tone
Your question was about 不, so my answer is only relevant for that character (and mostly for 一 too, like I said in the other thread). The third tone is something different and is written with the third tone mark, regardless of any tone change. If 你好 was written níhǎo instead of nǐhǎo, you wouldn’t know if 你 was a second tone or a third tone turned into a rising tone. And if you saw 沒有 (méiyǒu), it could be that 沒 is actually a third tone (which it isn’t, of course). This would be terribly confusing, so no one writes third tones after tone changes have applied, except perhaps in a lesson aimed at teaching this specific thing.