Two words meaning "autumn-colored foliage" are koyo and momiji, both written with the same two characters, 紅葉. Koyo refers to the phenomenon of changing autumn colors, mainly when it occurs to the leaves of deciduous broad-leaf trees before the leaves fall to the ground.
At some point in time, the word momiji became synonymous with the maple tree (kaede), the most beautiful of the autumn trees.
The species of maple generally determines the color the leaves will change: red, yellow or brown. Although the word koyo literally means "red leaves," it is used to refer to all the colors of autumn leaves. The word oyo refers to yellow leaves, and the word katsuyo refers to brown leaves specifically.
The koyo season in Japan typically begins in mid-September in Hokkaido to the north, gradually spreading to the southern end of the Japanese archipelago in about 50 days. The change occurs when the low temperature at dawn is about 6 degrees or 7 degrees Celsius, and the viewing season lasts for 20 to 25 days. The exact timing to see the leaves fluctuates somewhat from year to year.