About 12,000 litres pooled on to the carriageway following an HGV crash on the M606 in Bradford on Friday.
The Environment Agency said "no significant impacts" had been reported and becks around the motorway were "running clear".
It added "some of the pollutant is being washed through the system and can be seen in the [River] Calder."
Ash Milnes, from Dewsbury, tweeted that he had seen paint in the Spen Beck, near the town.
Its course runs close to the M606 and the site of the crash, before it flows into the Calder near Ravensthorpe.
The crash closed the southbound carriageway for much of Saturday to allow for a clear-up operation.
The agency said it had taken water samples for laboratory analysis and was assessing "watercourses downstream".
Officers would carry out further assessments on Monday, it added.
A quantity of white paint which was spilt in a motorway crash has polluted watercourses in West Yorkshire.