一 + 白 = 百。
You can see 白 is a candle here.
If you have a candle，your room will become bright，which is white，so 白 means “white”.
And in Chinese，The pronunciation of 白 is similar to 百，so we add a line on top of 白 to create 百.
You can remember this kanji by “It took me one dollar to buy this candle，and I sold it for one hundred dollars.”