I've had this patch of Sir Walter that loses all it's colour on and off for over 2 years. It's literally losing it's green and turning white, always in the same area of lawn. The problem can spread 5m or so, then, suddenly right itself and be fine for a few weeks, although in the very corner it always struggles and you can see in the pic the grass has died and clover is growing.
It's not lack of water or sun, I've tried fertilizer, grub spray and a grass fungaside non of which seems to work. My feeling is a fungal disease however googling the problem comes up with really nothing.
On the lawns I mow I have seen small patches like this from time to time, it's really got me stumped .