You wouldn't. To my knowledge, there's no way to color code the BCC
recipients different from the primary recipients.
If you click on outbox and then on the Organize icon, you can get different
colors if you're the only recipient, but I'm not sure how that applies to
BCCs. If it's sent to just you and three other BCCs, I'm not sure if you'd
see the color for only you or if you'd be aware others got it.