do you find 3.5 - 5g to be really heavy?
as far as changing colours is concerned, i've never been too concerned about colour. flathead are a pretty dopey fish when you think about it, as long as it looks like food they will take it. for flathead the most important thing is getting the plastic into the strike zone; within a meter of the fish. if you think they are around but not biting, i would slow the retrieve. even if it means ur plastic is sitting dormant momentarily it might just give the flatty a bit of time to decide that it does actually want to eat it.