Canvas borders are not supported in color mode except RGB

Photoshop only supports canvas borders in RGB mode.

Working in RGB mainly (and with a gray background anyway) it affects me little but I understand the original poster’s consternation about the distinction as it does not seem technically unavoidable while offering no apparent benefit. 

It's an alternative, but not a workaround. I often use the cmyk mode and customized a white background like print paper. I can't even do the cropping properly. 😕😕

The post together on the kr community says someone can't recreate it.

link

If you're trying to add canvas borders in Adobe Photoshop but it's not working, it's likely because your document is in a color mode other than RGB (such as CMYK, Grayscale, or Indexed Color). Photoshop only supports canvas borders in RGB mode.

How to Fix It:

1. Check Your Color Mode – Go to Image > Mode and see if it's set to something other than RGB.
2. Convert to RGB – If it's in CMYK, Grayscale, or another mode, select RGB Color to enable canvas borders.
3. Retry Adding Borders – Now, go to Image > Canvas Size, select a border color, and it should work properly.

If you're working on a print project that requires CMYK, you can switch back after applying the border.
I faced a similar problem while wokring for my website Magis tv para pc , i followed the same pattern and it worked for me.  Hope this helps! 😊 

»Boundaries are supported in rgb mode, is there a reason why not in other modes?«

None comes to mind, that’s why I added my vote to the Bug Report. 

»There is a problem that other operating systems are reproduced too, and someone says this is no problem.

I think he's lying.«

I explicitly mentioned that I can reproduce the issue on Mac, so who said what exactly? 

I want to know how my output looks when printed on white paper. Boundaries are supported in rgb mode, is there a reason why not in other modes?

There is a problem that other operating systems are reproduced too, and someone says this is no problem.

I think he's lying.

Only RGB mode supports boundaries. Is it verrrrry hard to support boundaries in other modes?

OK, that's the on screen canvas border, not part of the image.

And yes, you're right. It does disappear when you convert to CMYK. I have no idea why...

I can reproduce the issue (Adobe Photoshop 26.3.0 on  Mac OS 14.6.1); the setting from Preferences > Interface > Border does only seem to apply to RGB images. 

But … 

If the work is print-oriented I would discourage using a white background. 

The second lightest Color Theme and a light gray background should be fine. 

Apart from the rgb mode, the dividing line on the canvas disappears.