I have made hundreds of floating baits using the floating "bubble" additive, and have never experienced the baits being too soft. If anything, they were hardened up using the additive.
I use mostly soft plastisol. And yes the additive does wash out the color. One color I've made a lot of floating baits with is black. That should be the easiest color to make, but even black requires a lot of colorant to make it truly black, or you end up with grey of a washed out black.
I'm using a lot of additive to make baits that will float high on the surface to float over lily pads and such. Some other bait makers just make their baits float enough to suspend.
Either way plan on using a lot of colorant.
And if you want your biats to float. test the first couple of baits in a bucket of water with a hook in it. Once on the water with floating baits it's too late to find out they don't float.