I have "Selected Text" color set to a default value in vs 2010 RGB 51,153,253. But really in text editor it is RGB 173,214,255 that is slightly darker. The same things are with some other colors, e.g. Resharper Dead Code.
What can it be a problem and how should I solve it?
It's because Visual Studio is blending the selection colour with the other colours that are being displayed (that is, the white background). For example, if you look what happens when you've got a breakpoint set as well, it makes a bit more sense:
You can see it's actually blending the blue colour with the red of the breakpoint. This blog post describes the feature a bit more (but doesn't actually go into all that much detail... I couldn't find anything better, though)
Part of the problem is that the Tools | Options dialog does not display the actual colors that will be used by the Editor. VS mixes in an opacity value that can not be edited from the Options dialog. See this connect bug that is closed as by design.