Remember the difference between "color" and "paint". Color is a property of a square, and paint is something that overrides the default color of a square. The Star does not paint any squares, it simply specifies that the square with the Star and all bordering squares should have the Yellow color, and thus those squares are not subject to fading out in two weeks.
The reason those squares are not Yellow is because they are all painted (with Uncolored paint, as silly as that sounds). Rule XV-4 states that paint on a square overrides any other rule that specifies the color of a square. If paint thinner were applied to any of those squares, the paint would go away and the color would return to Yellow (until the Star moves elsewhere).