I like Obsidian with it too. But I like contrast. I never noticed a beige tint to Midnight Rose splashes. I could see GP being to close to the color or off color for the rose being the reason it was okay. Not enough contrast.
But I still wonder if you will be happy when the rose is not in bloom. When planning the garden you might want to consider what will look good when the rose is not in bloom.
I would consider what is going to compliment the rose and what view you will have when the rose is not blooming Spring and late Summer. I have Amber Carpet rose and it just started to bloom last week.
I am saying maybe you need the Okay color to compliment Obsidian when the rose is not blooming.
If you see what I did in my post Midnight Rose with a Rose.
The third pic I have Midnight Rose, Georgia Peach and Southern Comfort. So when the roses are not blooming they compliment each other but the when the roses are blooming they compliment the roses.