Why is 白鯨 pronounced hakugei but 白 is shiro?

Question

I was told that it changes because two kanji were put together (this sometimes happens)

is this related to On'yomi?

Answer 1

Kanji can have a number of readings. These are generally classified under the category of on'yomi and kun'yomi, where the former has its origins in the original Chinese pronunciation of the character at the time it was borrowed and the latter is the native Japanese reading given to the character.

There is no easy way to delineate when which reading is to be used. Some kanji have multiple readings, for example:

行

• This character has quite a few on'yomi: ぎょう(呉)、こう(漢)、あん(唐)。
• It also has a number of kun'yomi readings: いく、おこなう

The kanji in parentheses are indicative of the time that particular pronunciation of the the kanji were borrowed.

Just because two kanji are put together doesn't necessarily mean that they'll be read using the on'yomi, for example 仕事{しごと}. And, just because a character stands in isolation doesn't mean it'll be read using kun'yomi, for example 例{れい} (should I say pun intended?). There are even a few where kun and on'yomi are mixed 場所 {ばしょ}. And then there are the ateji reading of compound characters that have nothing to do with either kun'yomi or on'yomi such as 亜米利加{あめりか} and 仏蘭西{ふらんす} but also words like 沢山{たくさん} and 多分{たぶん} whose kanji might be using the on'yomi but the meaning of the compound has nothing to do with the individual meaning of the characters.

Essentially, how a kanji is read depends greatly on context and intuition. Perhaps instead of intuition I should say experience. As you learn to read more and more Japanese you'll get a better feel for when which reading should be used. This can be both frustrating and a fun adventure; I choose to view it as an adventure--it's part of what can make Japanese, as a written language, quite a treasure.

Answer 2

Yeah, this is related to on'yomi. Generally speaking, when multiple kanji are put together in one word then it is usually read with on'yomi. The しろ is the kun yomi and はく is one on'yomi for the kanji　白.

http://jisho.org/search/%E7%99%BD%20%23kanji

Similarly, 赤　(red) is red あか by itself, but often as せき when in compounds such as 赤十字　せきじゅうじ The Red Cross.