redirect sub domain in htaccess file
The piece you are missing is RewriteCond.

This should redirect just the subdomain

RewriteBase /
RewriteCond %{HTTP_HOST} ^red\.sitename\.com$ [NC]
RewriteRule ^(.*)$ http://sitename.com/$1 [L,R=301]


Note also that the separator needs to be escaped as \. to prevent it from being used as a single character match in the pattern.  The [NC] also tells Apache to ignore case during the match.