However, when Hollywood decided to make a movie based on the book, they saw fit to change the color of the mouse. In the book, he's clearly a brown mouse.
But in the movie they decided to change that to white.
Because Hollywood can only make movies with white heroes. Even when it's a white mouse.
They also changed him from being a biological child to being adopted, because realism is important with a movie about a talking mouse who wears clothes and has opposable thumbs.