Set Page level transparency blending color space to Device RGB

Question

My requirement is to set page-level transparency blending color space to Device RGB. I am trying to use pdfBox to achieve this. Screenshot is from the Adobe Acrobat reader (Print Production -> flatten Previewer -> change page level transparency color space)where you can set the value from the dropdown.

I tried to set blend mode using PDGraphicState of PDFBOX. Is it right way to achieve page level transparency showed in screenshot?

PDGraphicsState gState = new PDGraphicsState(page.getArtBox());
gState.setBlendMode(BlendMode.OVERLAY);
PDExtendedGraphicsState pde = new PDExtendedGraphicsState();
pde.copyIntoGraphicsState(gState);
final COSName blendMode =page.getResources().add(pde);

But this is not working. I have some other code which uses "PDExtendedGraphicState"

which will be added to page using page.getResources().add(graphicsState) But PDExtendedGraphicState does not have any method to setBlend Mode. So I created new PDExtendedGraphicState object and did "copyIntoGraphicsState".

Am I missing something or the approach is wrong?

Thanks in advance.

EDIT
Before Open file in Adobe acrobat Pro DC. Open : Tools -> Print production -> output preview. Try to open and close output preview. you can see color shifts. Before

After Open same file (Before.pdf) in Acrobat Pro DC. Open : Tools -> Print production -> Flattener Preview -> Look for Page level transparency blending color space(See the screenshot) -> change -> select Device RGB from the dropdown -> apply. Now you will get after.pdf which I added here. After this if you open output Preview as stated above you will not see shifting of colors.
After

Answer 1

To set DeviceRGB as transparency blending colorspace for the document. We have used:

group.setItem(COSName.S, COSName.TRANSPARENCY);
group.setItem(COSName.CS, COSName.DEVICERGB);
page.getCOSObject().setItem(COSName.GROUP, group)

This solves problem. Thanks guys for suggesting different approaches.