In reply to pheotleyr:
The colour is going to depend on the amount of sediment suspended in the water. At the moment we've got neap tides (small tidal range) meaning the turbid water from the Bristol channel doesn't push so far up river (if it does it may just be at the very top of the tide). We've also got a high flow coming down the river from all the rain, but most of the sediment has now dropped out of that as it's not rained heavily for a few days. I'd speculate, also, that as things have warmed up a bit, and the water will be heavily loaded with nutrients from agricultural run-off, that the green tinge may be algae.
The water in New Cut also looked a touch green today. At least a greeny sort of brown.