track changes

I work in a law firm. Documents are often emailed to and
from this firm to another law firm with track changes
showing. When we receive a document from another firm
with track changes, we sometimes cannot "accept"
or "reject" these changes. In order to show our "new
changes", a new color (other than what is in the
document) has to be applied to our changes and
forwarded. We do not know if they will be able to accept
or reject our changes or if they too will be locked out
of it. Are we locked out due to Protection being turned
on at the source of the document? Is there a way to
Unprotect it from our end so that we can respond to their
changes and include ours? Or what would be the proper
way to handle this? Should we just be adding our changes
in another color and leaving their changes as is?

Also, is there a way to choose a particular color such as
blue for inserts and red strikethrough for deletions
rather than having the different colors indicate authors?