Nitrogen is used throughout the life cycle of a cannabis plant. It will be used more so in "Veg" to build stems and leaves, but will also be needed in smaller amount during flower. This is because plants use nitrogen to help them make food with photosynthesis. Need help with nitrogen issues? Start a topic here
If your cannabis plant is looking pale, dark, or has rusty spots on the leaves, it may be suffering with nutrient deficiencies. You can fix these problems with a few days usually, just ask for help if you're unsure.