mod_rewrite help

Here's my situation.

When a user access the site domain.com it redirects to /portal which displays SKIN1 by default.

When a user accesses the site domain.com/portal?branding=SKIN1 it displays SKIN1 by default as well.

When a user access the site domain.com/portal?branding=SKIN2 it displays SKIN2 as its supposed to but as soon as that same person goes to domain.com or domain.com/portal it shows SKIN2 by default and no longer SKIN1

I need a rewrite rule or something that will say /portal always points to SKIN1 accept for when accessing SKIN2 and only show SKIN2 when its chosen.

Let me know if this makes any sense?