Yes! Some bitches never show any colour, and some are very clean and you just don't notice. If the vulva is swollen and the bitch is licking herself, or male dogs are showing interest, then you need to be very careful. Bitches can be very clever when it comes to finding themselves a mate, and will escape even if they normally wouldn't dream of it.
You need to watch them for at least three weeks (I had a bitch get mated on day 28 once).