It seems to be a hard-coded colour, I think there's a problem with your graphics display. What graphics card does your system contain? If you've got a Quadro or FirePro, you should be able to search the SolidWorks website as there are approved drivers recommended for approved graphics cards.
If you're on a Geforce or Radeon card, you could simply try installing the latest available drivers for your graphics card and just hope you get lucky.
It's supposed to be a shade of grey, as shown below.
Alin ,once you get reply ,let me know please ,
yes, it is useful transparent option in my case ,
It would be not easier to create a necessary surface several splines? Rather than to change a simple surface through freeform
Sorry for the delay.
Looks like the issue is caused by selecting one of the Blue, Green or Orange Highlight color schemes in Tools>Options>Colors.
To show these transparent faces correctly, you can hit the Reset button on that options page and choose just to reset the options on that page only.
This issue was previously reported as SPR 633551 - Selecting color highlight scheme will change the transparency of solid bodies to black when using the hole wizard or freeform.