I sure do take my good, sweet time when it comes to foundation swatches, don't I? My apologies; I should've had these up months ago. Hopefully, they're still helpful! And if it makes you feel better, neither product has a shade match for me.
As per usual, there's a shot with flash/artificial light (top) and a shot in filtered daylight (bottom).
Up first is the Smashbox Camera Ready BB Cream. Sorry about the slightly meager, sloppy swatches; the samples were pretty tiny. I swatched these on my inner arm, which is roughly N5, so I'd guesstimate that the lightest shade--Fair--is about NW15-20. All of these shades seem to have incredibly pink/red undertones.
This is the Becca Radiant Skin Satin Finish foundation, which I've actually tried in Shell. I swatched these on the top of my arm (so yeah, sorry about those arm hairs...), which is about N15 right now. Based on that you'd think Shell would work for me...but it's far more yellow than it looks. When I tried it on my face, it looked downright GREEN, and I washed it off after all of 15 minutes. Porcelain is uber-pink, but quite fair. I'd guesstimate that Porcelain is about NW10 (which very strong pink undertones) and Shell is around NC15 (which very strong golden-yellow undertones). Buff and Buttercup are almost identical in shade depth; the major difference appears to be the undertone.
Also, the Becca samples were super-generous, so huzzah! :)