en route to Mt Cook is Lake T, which is bright blue because of the suspended rock flour.  It scatters the blue light just the same way as the sky does.

Further up the feeding river is grey because there is so much suspended sediment, scattering most of the light back.  Same reason clouds are white and not blue.

When the lake is stirred up, as happened next day, the water has a big particle load the scattering becomes full, and the water is grey.