I am putting up a black metal railing on stairs and loft

Since you're modernizing, nail up 1/4" sheetrock over the diagonal paneling. It is dated and will look dated even if painted.

Thank you - and stay with my thoughts of white ceiling and black beams or paint it all white?

Yes you could do the typical paint it all white, or you could create a more exciting interior with staining the wood using a transparent stain.

I would also reconsider adding the black metal railing. See this wood railing below. It would be less expensive and more visually effective.

https://worldofarchi.com/2013-12-30-rustic-chalet-interior-design-ideas/#google_vignette

That wood in the inpos is beautifully stained but none of it is diagonal. And diagonal boards for those stair rails in front of them as suggested is a lot of angles.

You could paint the current rails black. I would try that first.

If the railing and wall are stained the same color, they should fade into one another.

Just like they do in this photograph. The wall planks are horizontal and the railing is at a 45 degree angle.

Here is your room with the stained walls

I wouldnt paint it. I would try to stain it all, and leave the wood.

I like the pine(?) ceiling. I wouldn't paint it.