As nouns the difference between tao and tan

is that tao is the essential principle underlying existence; ultimate reality while tan is a yellowish-brown colour.

As an adjective tan is

of a yellowish-brown.

As a verb tan is

to change to a tan colour due to exposure to the sun.

As a numeral tan is

the second cardinal number two, formerly used in Celtic areas, especially Cumbria and parts of Yorkshire, for counting sheep, and stitches in knitting.See Wikipedia article on Yan Tan Tethera

As a proper noun Tan is

an English surname derived from the transliteration of various Asian surnames, particularly originating from China and Vietnam.

As an initialism TAN is

Total Acid Number, the measure of a lubricant's or crude oil's acidity.