At this time of year, purple leaves are not uncommon. Mine, growing in the ground outside all winter (started from seeds), is now mostly purple. Phosphorous isn'what it needs. Phosphorous is used for getting blossoms and for encouraging rooting. The lower leaves will die out and sometimes its due to excessive moisture, and/or poor draining. If anything, you would want to add a little nitrogen to assist the foliage. Here, in my Z6 is one of the most hardiest of herbs, as I never expected it would survive our winters.

My thyme is potted (still outside) and looks much like that...but don't worry it will come back to its normal color in a few weeks.

The answer is similar to your mint question. Under cool conditions or during dormancy thyme and other plants often have discolored leaves. It has to do with the sugars in the leaves or lack of them. It is a similar process that happens with trees in the fall-the colors are yellow and red and shades in between. Just leaves don't fall of with thyme though. Once growth resumes, everything is back to normal. Though you may have to remove some of the older leaves-new green ones will replace them.