The government has yet to find the source of pollutants which turned the Shing Mun River in Sha Tin blue earlier this month and then turquoise this week.
The Fo Tan tributary of the river was seen taking an unusual colour on Monday, reported local Chinese-language newspaper The Sun.
About 800 metres of the waterway was partly grey and partly turquoise, according to The Sun. The water also gave off a foul smell, although no dead fish were spotted.
Last week the river turned bright blue, with Apple Daily reporting that chemicals from a nearby industrial zone may have been the source.
The Environmental Protection Department said at the time that they had taken water samples from the river for tests.
The EPD told HKFP results of the tests are “expected in the next couple of days.” In an email reply, a spokesperson for the EPD said officials have inspected waterways and some industrial buildings in Fo Tan but have yet to determine the source of the pollution.
The spokesman added the EPD conducts regular checks on water quality in the Shing Mun River.