I have a notoriously black thumb, much to the chagrin of my Master Gardener mother, and I can't even keep aloe alive. So I asked very specifically what to do with my coleus when I purchased it. It is lush and thriving, but has lost its color!! It was a variagated yellow and brick red when I bought it, and now it's almost entirely green. The man at the farmer's market said to keep it out of direct sunlight and to water every two days. Now I'm second guessing if that means every other day...or if it needs more sun, or what. I did not repot it, as I've had terrible luck with that in my plant-past, too. Any suggestions?
There is certainly no need to be too tough on yourself. Chances are you just need to know a little bit more about the plants you are trying to grow compared to your light situation at home. With a little more knowledge you'll be able to grow the right plant in the right place and you will see those thumbs of your change color in no time!
So let's get to the coleus. This plant is considered an annual for us so that means it will not survive the winter outside, but can be kept going in a container inside. If we were talking about the plant being outside in a garden then the grower's advice would be accurate. Coleus (botanically known as Solenostemon scutellarioides) is a sun to shade tolerant annual and can possibly burn and/or bleach out if exposed to an excess of 5-6 hours of direct sun. However, if you have it inside in a container, you want to try and give it as much direct sun as possible. That is the reason for the loss of non-green color. The plant had that great variety of color because it was getting enough sun before. In its present situation it is getting significantly less sun so it is adapting to survive in the environment. The green in plants leaves is chlorophyll, cells devoted photosynthesis, the process of turning sunlight and water into carbohydrates so the plant can grow. Given less light, the plant has been forced to produce leaves that are all green to max-out the plant's ability to photosynthesize and hang in there. To give a personal example, I have a plant called a croton here in the office. Naturally the plant is wildly multi-colored as it is a tropical plant used to warmer and sunnier climates. However, I have it here at my desk growing under fluorescent lights. The full spectrum of light the bulbs emit is enough to keep the plant going but is not the same as a direct sun situation. Therefore, I instead have a plant that is now, like yours, entirely green.
If I were you I would try and move it to a sunnier spot. The plant will adapt to getting more sun and hopefully the interesting color patterns will return. You might, if possible, move it to a sunnier spot in stages. Move it to a brighter spot and care for it for a couple weeks. Then move it to the sunniest spot and continue to care for it there. Moving the plant in stages will help to prevent the green leaves from getting too sunburned or scorched since now they are used to less light. As another example, I move my succulents outside for the summer. At first they might get a little fried,and I might lose some of the older leaves, but they usually bounce back pretty quickly. If you lose some leaves in the process but the plant is still producing new leaves then don't fret it too much.
Otherwise, keep the soil moist this time of year and then water a little less in winter. If the coleus has put out a lot of new growth then you should definitely repot it so that it continues to get the nutrients it needs and has room for its root to grow as well. Usually you want to repot plants when you get them as growers might have them rather pot-bound in order to sell you a full looking item. You only need to repot it to a container a couple inches larger in diameter. Clay will dry out more quickly; plastic will help the soil to retain moisture for longer. A standard brand-name potting soil will be fine. If that makes you nervous give me a ring and either I can talk you through it or we can set a time to bring the plant in to HSNY and we can repot it together.