I've never heard of that old wives tale but I have however heard that (depending on the mother's body type) you can tell by looking at a woman what gender baby she is having. If the baby is held mostly in the lower part of your belly then you're suppose to be having a boy and if its held mostly in the middle its suppose to be a girl. Its a fact that isn't 100% and therefor isn't proven but a lot of the pregnant women I know have attested that its true.
They get darker whether you are having a girl or a boy. It is the hormones in your body during pregnancy that causes these changes to your body. You may also notice a dark line from you belly button going down to your pubic hair line, another proof of hormones and pregnancy.
There is no way to tell if you're having a boy or a girl other than through ultrasound. There are no specific symptoms that one gets while pregnant with a boy vs girl or vice versa.