Is it possible, to switch back to the old style, where the tabs weren't yet fully colored. It looked very good before, but now all colors are much too candy-like. The readabilty suffered too, although i know other users prefer it this way. Maybe we can have both options, colors just at the top, or fully colored?
You can change the colors. They are completely in your control.
I know, but i liked the old style much better. The way it is now, it never looks good, even if i apply a very slight color. Another problem is, that the tab color doesn't spare the lock symbol, but paints over it, giving the symbol a "milky" appearance.
I guess, i have to turn the tab coloring off then.
When i change the colors, the old colors will all return, when i reload Opus. 
Ok, found it. I have to save it for the style i'm using. But i think, once we set the color there, in the tab's menu, it should always stick automatically.
The adjustments that used to be done to the colors before were not magic. You can make the same adjustments yourself. Just move the Luminance slider up, basically. That is what the code used to do to the colors, and now it doesn't because people wanted full control over them.
How the lock symbol is painted has not changed.
Are we talking about tabs on the left, or bottom/top? It would help to specify. Some screenshots might make sense as well.
Tabs on the bottom look fine to me:
Tabs on the left, if colors and lock are combined, there could be a bit more margin between the lock and the color, although it still does not look "milky":
Then you have changed them in a way that is only temporary. More information is needed. What are you actually doing? There are several ways to set tab colors, and some of them need to be saved if you want them to be saved.
No, that isn't how any other detail of styles works. You're jumping to design changes because you got a bit confused for a minute. 
When i change a current color, it's not represented in the color mixer. I have to make a similar color, befor i can adjust the brightness. Maybe this can be approved, so that the left slider would have the current color. I have the feeling, that it's already supposed to be that way, but it wasn't, when i changed the colors a few minutes ago.
There seems to be a slight hue, depending on the underlying tab color. But it could be some optical illusion. It is known from those optical illusion web sites, how a neighbouring color can influence the perception of another color. Anyway, the left lock symbol apperas to be a bit reddish, compared to that on the right.
I have the tabs in the "classic" way, on the top, as in the image.
Yes, i had to save the whole style. Makes sense, to also have temporary tab colors.
I was a bit surprised about this change, yes. The release notes don't even mentions it. If possible, maybe we could have an option to also use the older style, because it allowed me to use more saturated colors. With the new style, too much saturation looks too intense.