I have had a child almost 10 yrs ago and I can still get a drop of milk out. During foreplay it does increase by just a bit. It rarely comes out without some help but it is only a few drops. I was told by my doctor that the color is the indicator of problems. Mine is a pale yellowish milky color and was fine.
Are you breastfeeding? If so then yes, you can leak milk during foreplay. The hormone that releases all the milk is called oxytocin and it's the same hormone that's released when you climax and have an orgasm.
If you aren't breastfeeding and you are leaking milk then I would suggest you go to the doctor and get a checkup. You may have a benign tumor on your pituitary that's making you produce milk when you shouldn't.