When you start learning Kanji, this will start to be more apparent as you will need to learn roots of words so that you can add the proper hiragana endings to the words. It gets even more crazy as you can use words from one part of speach as another part of speach by just changing the ending.
Shiro is also one of the 108 Stars of Destiny in the video game Suikoden II. It's a white wolf.
It has nothing to do with female or male. It’s not a pronoun either. See other people’s responses above. あか is the noun, あかい is the adjective. The only example I can think of where the colour may be used as a noun is in a sentence like 好きな色は赤です （すきないろはあかです） = my favourite colour is red.