This is the 657th most frequent Japanese word.
赤
"赤" (aka) means "red."
The word 赤い (akai) is used as an adjective to describe the color of the flower.
この花は赤いです。
This flower is red.
The word 赤く (akaku) is an adverb form of 赤い, describing the state of the leaves changing color.
秋になると、山の葉が赤く染まります。
When autumn comes, the leaves of the mountain turn red.
The word 赤 (aka) is used as a noun to refer to the color itself.