Here's how the phrase came to be:
* White rice: White rice is a staple food in many cultures. It's simple, plain, and easy to recognize.
* "Like": The word "like" is used here to emphasize the similarity between the obviousness of the thing being described and the plainness of white rice.
Examples:
* "It was like white on rice that he was lying – he was sweating and couldn't look me in the eye."
* "The answer to the question was like white on rice – everyone knew it."
* "She's got a crush on him, it's like white on rice!"
Essentially, "like white on rice" is a way of saying something is as clear as day or as obvious as the nose on your face.