Microwave Harming My Plants?

I must admit that I hardly know what I'm doing. I've never had a green thumb, but recently I planted a few types of herbs and they have been doing well for about six months. I have them inside my home and due to lack of space, I placed them on top of my microwave oven. Most of the herbs are doing fine (parsley, peppermint, chives) but my sage and basil have both gotton a grey or rather, a silver color to their leaves. They are all in front of a window and have a 24 hour plant-light directly above them. All of the plants get sufficient water. I did recently replant them about two months ago since they had outgrown their original seedling pots. What could be wrong with them?

The two plants that you have problems with are the two requiring the most light. The fact that their leaves are becoming silvery could mean a bad spider mite infestation and the mites like these plants because in the lower than optimal light their leaves were the most tender. Check the leaves with a magnifying glass to see if there are tiny translucent "creepy crawlies" on them. If yes spray with a neem oil and dishwash detergent solution after first rinsing the leaves in a stiff spray of water over the sink.
Alternatively, basil does not like repotting and on top of it is an annual. It may have simply decided that life was better over after the stress and its old age.
Sage likes it on the dry side. Keeping it as wet as the other plants may have rotted roots and set the plant back -even more so after repotting.