在我走路回家的时候，看了很漂亮的日落，哇，红色与橙色混合，好神奇啊！其实在我看日落的时候，问自己"为什么在日落的时候，天空看起来红色，但是在白天的时候，天空看起来蓝色" 很有趣啊！

我们午餐吃了芋头跟蔬菜和米饭，其实这是第一个次我们自己煮芋头，非常好吃。

上个星期我们去了大城市买了一些衣服，他们那里有很多衣服有折扣，我们看到很多不同的东西，可是然后觉得很饿了，所以去吃午餐，好玩唷！

今天我的工作很忙，但是我完成了最重要的事情

*It's redundant but I won't say it's absolutely wrong. If you are speaking it's a really common filler :p

**It's common to omit the subject in a Mandarin sentence, but I prefer to keep it here.

***I only use 做 + dishes. Taro is a plant (ingredient) rather than a dish. Or you could replace 做 with 煮. It's correct to say "煮芋頭".

Basically there aren't many mistakes. Good job! I do have some other opinions on the wording, phrasing and the structure, but that would involve too much personal preference.

As for your question about sky colors, you could search on the internet with the keyword "Rayleigh scattering" :)