The fish can no longer see the lure clearly and must use its other senses to detect its prey. Coloured water can be caused by a fresh in a river, run out tides from muddy bays, wind and the general nature of most rivers. At this point I would change to a reddish brown coloured lure, with a slow pulsing action. No better example exists than the famous range of Nilsmaster from Finland. The simple slow action Nilsmaster, well resembles the swimming motion of most baitfish and will take flathead in a variety of locations.
Size is of particular importance.
The rule here is that small lures, like bait fishing, will take more fish. To take big flathead however the use of bigger lures is a must. Don't be afraid of a lure's size when you first see it in the box. Just imagine the actual size of an average six inch or 150 mm baitfish, then look back at the lure and you will see what I mean. The depth which a particular lure will dive to is also of great importance, as is the ability of the lure to float or sink.