Nowadays, we don't really get to receive those conventional card invitations since everything can easily be sent thru email, online video or thru Facebook. The only invitations that I get to hold are the school party invites from my kids' classmates.
A few months from now, one of my cousin would celebrate her 18th birthday. Of course, aside from the e-birthday invitations she's going to send, she would also be creating a card invitation. With her red and black motif, a black card stock, would be appropriate for the occasion.
Sending online invitations are faster and easier. It also reduces paper waste and frees up your budget. Still, not everyone, particularly the oldies, has an email, Facebook account or Internet access.
Even if most invites are being sent online, it is still a good idea to make use of those traditional invitations especially during special occasions - like wedding, 18th birthdays, etc. Personalized party invitations are more formal, make your guests feel more precious and your event more important.