All he did was throw paint balls at the canvas.
我的理解是
all he did 即 all that he did 是定语从句，
throw paint ... 是表语从句嘛？为什么不是一个完整的句子？
------解决方案--------------------
这个句式我见过哈，这是一个主系表机构的句子，典型的特殊省略句式。。
all he did 是主语，all可以引导名词性从句，像what he did一样。throw paint balls做表语，本应该前面有to的，也可省略。
一个典型口语：All I have to do is learn English.