しろ is Kun yomi (original Japanese word). When you see 白 and Hiragana combination such as 白い, you most likely read the kanji in Kun yomi (shiro).
はく is On yomi (based on original Chinese sound). When you see 白 combined with other Kanji such as 紅白, you most likely read the kanji in On yomi (haku).
Please read the article on the following site for further explanation of the difference between On and Kun. It's well written.