Yellow, brown, discolored, blotchy leaves
Noticing discoloration in leaves can mean multiple issues.
The first thing to do is to isolate your plant from your other plants. Be sure to inspect the plant and keep it quarantined until it is well.
A causation can be from under-watering, check to see if your plant soil is very dry and you see some roots are crispy. This can be followed by dry or shriveled stems.
Be sure to add water to your plan. This can be done by giving it a 15-20 min soak by placing the container (with drainage holes) in a bowl and adding water to the bowl. The water should soak up the soil. Once the soil is moist, remove from the bowl of water and still quarantine the plant away from the others just in case.
Leaves have blotchy dark spots through due to fungal infection
Bacteria & Fungus
The plant may have a bacterial or fungal infection. If the soil feels moist/wet, this can be an infection your plant may have contracted by having too much water or moisture. Add a bacterial or fungicide to the soil. If the plant is very damaged try removing all the soil and wash the roots well with soapy water, remove damaged (blackened) roots and repot in new soil. Throw away the old soil completely because it is contaminated.
Bacterial or fungal infections can also be found only in the leaf if the leaf is constantly wet. Make sure to not over mist or splash your plants with too much water.
Quarantine the plant and check up on its status frequently.
Chlorosis can also be an issue. This is a yellowing of the leaves with green veins. This typically means the plant is nutrient deficient.
Try out a fertilizing schedule by adding a specific fertilizer to your plant type, with a regular schedule.
Air Flow, Temperature, & Humidity
A lack of air can cause the plants to discolor as well. Allow the plants to have a good amount of air flow, like putting them in a bigger room or an open space.
Make sure it is not getting draft (dry cold air), like from the window as this can cause it to discolor as well. Try maintaining a humidity of between 40- 60%.
Plants can also be stressed in too high or too low temperatures, this can cause the plant to have discolored leaves that may fall off. Try to keep the temperature consistent. Most house plants enjoy 70-85ºF climates.
The plant may have a lack of light. It can start to either darken, such as a variegated plant losing its colors, or lighten into yellowing and falling leaves. The plant is doing this because they are not getting enough light to make “food” for themselves. Make sure the plant is having an adequate amount of light.
In contrast, too much light is also a problem. This can bleach the them making them seem dry with pale-yellow or white leaves. Try to move the plant to a less bright location.
If none apply, and the plant has had the leaf for a while and it started to discolor out of nowhere, then it may be the natural cycle of the leaf dying back.