Had the very same thing happen to me just last week.

Epsom salts, as JSG mentioned, seemed to do the trick for me.

You can see the issue I had and I took some some pics in my latest journal update here: LED Logger

I was also advised that as some leaves were too green I might have too much nitrogen in the soil. I'll just stick to watering from now until the end of bloom which will be soon.

After applying the Epsom salts the issue has stopped. All new leaves have returned to green. I might add a little more and try the foliar feeding as JSG mentioned.

Hope it turns out ok. You can get Epsom salts really cheap in any chemist.

Take care