You can understand that the central fringe itself is white. Fringe width of individual colours is directly proportional to (approximately)  the wavelength of the colour. To understand why, it s easy enough. Central fringe is the place on the screen where there is zero optical path difference(or phase difference) between any to waves simultaneously emanating from the two slits. Moving away from the central fringe means there is a difference in distance travelled by such waves to reach the given point. When you move more closer to one of them means you increase the path difference. When you want more change in phase difference for lesser path difference, then wavelength must be smaller. 2 pi times the ratio of path difference to wavelength gives phase difference.