yellowing 5 weeks into flowering

I have had a problem with yellowing of 1 plant since the third week veg,and now about half the plants are getting yellow.

My grand daddy purples(to the left),are starting to get a little yellow. If you look you can see it...

The hindu kush+skunk on the right,(the 1 in the middle on the left), has always been a little yellow,but the other 2 are just fine...

My nutes i am using

botanicare pureblend pro bloom,karma and sweet.

things i have tried and are using right now, Cal-Max,and pk13-14...
Still the yellowing contiues...
I still have 3 to 4 weeks left....
For got to add this is my first grow...

Thanks...

Yellowing the last half of flowering is pretty normal. the plant is using its last reserves of nitrogen to sustain flowering as long a s possible in the hopes it will be pollinated. The more yellow the leaves and such the less chlorophyl there is to have to cure out.

That is good to hear...That means that my GDP might get done before my Hindu...
In the second pic, You can see the gdp leaves starting to turn purple...

Thanks...