"I bought a pen, its color is blue"

Some grammar mavens would say that this sentence contains an error which is called a comma splice (link). You might rewrite it one of three ways:

1. I bought a pen. Its color is blue.
2. I bought a pen; its color is blue.
3. I bought a pen, and its color is blue.

JFTR, comma splices don't bother me, but they do bother English and ESL/EFL teachers I have known. I don't usually correct other people's typos or mistakes, but, in this case, it's for callithump's benefit.

Ceci n'est pas un seing.