More Lithops Trouble

[StarDragon]

There has been a sudden color shift of one side of one of them, with no evidence of decay or other ailments. The top of the darker, discolored side, is faded. I couldn't find any other conclusive information on this. I couldn't get the photo to come out very well, but the normal color scheme is pinkish copper markings on a brown top & pinkish sandy brown sides. The discolored section is a grey green, with a darker line separating the normal & abnormal colors, & the top of that section is a faded shade of the normal colors. The roots appear to be healthy & aside from the discoloration & aforementioned wrinkling, it appears to be healthy. What my be causing this?