。
The leaves have started turning red and yellow.
Literal
Tree-[possessive-の]-leaves [subject-が] red [or-や] yellow [to-に] became-begin.
や is the non-exhaustive list particle — 'X, Y, and (others)' — used instead of と ('and') when the listed items are examples rather than an exhaustive enumeration. Here 赤や黄色 implies 'red, yellow, and various other autumn colours.' 赤 and 黄色 are used as nouns with になる, and 始める attaches to 'become' as the aspectual auxiliary.