Let's say there are two files, "main" and "rename".
The file "rename" has a few lines with names for appending on 
the alternate rows of file "main". (according to the position in the "rename")

I has been using `paste` for concatenating lines from different files, but I was stucked on this case.

Is there any other ways of doing this without using copy and paste?
(Preferably with the command that can be readily used in Ubuntu Linux 18.04.)

file "main"

```
#
stars
#
twinkle
#
on
#
the
#
sky

```

file "rename"

```
yellow
white
green
red
blue

```

desired output

```
#yellow
stars
#white
twinkle
#green
on
#red
the
#blue
sky

```