Don't feed them and they'll go a nice shade of yellow.
Are you giving them CalMag? Low calcium can darken the leaves before spots start showing up. Too much N will do it too. Some Epsom salts is good to add for some extra Mg and S
If you just flushed it will take a bit for you to see any changes. Should start seeing some yellowing in the old fan leaves in a week if you got a lot of the nutes out. How hard did you flush them? 3x pot volume is what I call a hard flush and the last should be 1/4 - 1/2 strength nutes so they can carry on growing and budding well with some fresh food on the plate.
I just got some of those liquid ProMix nutes. Got an All Purpose 12-6-6 which should be good for veg and the Tomato, Veg and Fruits one at 3-6-12 which is about perfect for flowering. Have to try it against my AN nutes on a little side by side grow in HP. Not going to pH test those either to make it fair and square.
