I know plants need Nitrogen when they are turning yellow but what about when they are flowering? Is it still a good idea to give them ferts high in Nitrogen when they are flowering?
Just stick to your flowering ferts. Giving them more nitrogen now will make them focus more on vegetative growth and less on producing buds.
You are trying to walk a fine line. Hootskers, I hate to disagree, but I think there are circumstances where adding some veg (high N) ferts during the flower cycle could do more good than harm. If you are relatively early in your flower cycle (first 3-4 weeks) and you are getting severe yellowing of lower and middle leaves and even leaf loss already, I have mixed a fert batch of half flower nutes and half veg nutes to try to replenish the N. You can't lose all your leaves too early or that will hurt bud production too. But in general Hootskers is right, N retards bud growth, so I would not use full-strength veg nutes during flower nor use them in the second half of the flower cycle.