You could do a few things depending on your needs.
1) if you want any query string added, you can use [QSA] (Query String Append)
Code:
RewriteEngine On
RewriteRule ^index\.shtml$ http://someserver.tld/something.php [R=301,QSA,L]
2) if you want only to redirect if the value is red, you would do this
Code:
RewriteEngine On
RewriteCond %{QUERY_STRING} ^(red)$
RewriteRule ^index\.shtml$ http://someserver.tld/something.php?%1 [R=301,L]
3) if you want only to redirect one of a set of values (red or blue, split by a pipe) you could do this
Code:
RewriteEngine On
RewriteCond %{QUERY_STRING} ^(red|blue)$
RewriteRule ^index\.shtml$ http://someserver.tld/something.php?%1 [R=301,L]
Edit, if it's the same domain you don't need it, just /something.php
Edit2: I said [QSA] was optional on the last 2, it's not (although it could be).