You are hereLight Burn
Problem: Your cannabis plant can only withstand a certain amount of light. After a certain point, your cannabis will start turning yellow or otherwise exhibit signs of stress on the leaves near the sources of light and/or heat.
Light burn usually causes yellow leaves at the top of the plant directly under the grow lights (though it can appear on older leaves that have been exposed for a long time).
Light burn is often mistaken for a Nitrogen deficiency which makes wilting yellow leaves. Nitrogen deficient leaves fall off on their own, while light burned leaves are hard to pluck off. A nitrogen deficiency starts from the bottom of the plant and moves up, while light burn often is worse at the top of the plant.
Cannabis light burn usually affects the top leaves closest to the grow light
A Nitrogen deficiency creates yellow leaves at the bottom of the plant as the Nitrogen is sucked out of the oldest leaves to feed the top of the plant. On the other hand, light burn produces yellow leaves at the top of the plant under the grow lights because the leaves have worked too hard and/or too long from the light being too close. The leaves aren't able to keep up with regular plant processes.
Imagine sitting outside all day under a scorching sun, possibly for days on end. Even if you could handle it for a day or two, it might wear you down over days or weeks.
It's probably light burn if mostly just the leaves closest to the lights are turning yellow
With cannabis plants that have light burn, your leaves can become yellow or red/purple, possibly with brown spotting, often with burnt tips/edges and margins that stay green. Other problems, like nutrient problems, can trigger or make the symptoms of light burn a lot worse. Leaves may also appear generally burnt in places when there's too much light, especially when combined with heat or nutrient problems.
If you see light bleaching and unhealthy discoloration only on the parts of the plant directly under your grow light, or only on older leaves that are exposed to the light, it often means it's too bright for your plants and you should move your grow lights further away! If your plant is also having other problems, it is much more likely to be affected by light burn. A healthy plant can withstand higher light levels than a sick plant.
With light burn, often the inside veins stay green. Yellow leaves won't fall off or be plucked off easily, unlike a nitrogen deficiency where leaves fall off on their own.
If the lights are only a little too close, maybe just a few inches, the yellowing from light burn may happen slowly because leaves are dying early instead of right away. Because of that, light burn may first appear on somewhat older leaves, which can be confusing and make it hard to diagnose.
This cannabis seedling is being burned by too-close LED grow lights
This cannabis seedling basically grew up into the grow light! The heat from the bulb caused massive burning everywhere it touched. If a plant's leaves directly touches the lights, it leaves "burns" from the heat of the bulbs.
This plant was green and healthy through the vegetative stage under an LED grow light, but the leaves started dying soon after flowering started. The reason was the LED was too close.
These plants seem apparently healthy, but the top leaves keep getting lighter and lighter because the LED grow light was too close. If you don't realize it's light burn, the symptoms are inexplicable!
Light Bleaching Cannabis Buds
This is how you get "albino" buds. Light bleaching is most common with high-power LEDs and HPS lights that are kept too close to the tops of the plants. Basically, this is what happens when plants get too much light, kinda like how hair on top of your head can turn lighter if you spend plenty of time in the sun, except a "sun-burnt" bud isn't as strong!
Buds which have been bleached tend to be low potency or even have no potency (no available THC or other cannabinoids). Therefore you should avoid light-bleaching your plants at all costs!
Sometimes light-bleached cannabis will get mis-labeled as “albino cannabis” or “white cannabis” but the truth is that the white color is not healthy, so this is not a desirable trait (even if it looks pretty cool).
Often though, light burned buds look like they've been burned.
Light burned bud on top, healthy bud below
Solution: If your marijuana plants are getting too much light, try removing some of the lights or moving your grow lights further away from the tops of the plants. If you can't move the light further away, bend your plants over so the tops are further away or if your plant is still in the vegetative stage you might even consider cutting off the top of the plant to remove some of that height.
Reduce power of grow lights and/or move them further away from your plant
(How far away do I keep grow lights from my plants?)
It is unlikely for your plants to get "light-burned" from the sun when growing outdoors, and they definitely can't accidentally grow into the sun. Outdoor plants can show signs of light stress if plants were used to shady conditions and moved into direct sunlight without time to get accustomed to the brigher light levels. It also may be possible in extreme high light conditions if the plant is unprotected but in general cannabis plants like a lot of light.
When making changes to your plant's environment, it's best to make changes relatively slowly if possible. For example when moving a cannabis plant from indoors or outdoors you might consider giving the plant some shade for a few days before moving it into full sunlight.
Sometimes heat stress can look like light stress. When learning how to grow cannabis, it's best to try to keep things at a comfortable temperature at all times for optimal growth. If it's too hot for you, it's probably too hot for your plants. Outdoors, it's a lot harder to control temperature, but there are steps you can take to protect your outdoor plants from the heat including supplementing with sea kelp, partially covering them and making sure they're well watered.