InDesign Cloud Libraries can't be color managed?
Elements added to the CC Library from InDesign only allow CMYK Transparency Blend Space!! Is seems not possible to change to RGB.
Follow me through:
1. Create a new doc, RGB blend space.
2. Add a box, apply a gradient to it (any gradient, but I would suggest to you to use RGB strong values, 100% Red to 100% Green).
3. Select the element and drag it to the CC Libraries panel.
4. Close and discard the new document created.
5. Go to the CC Libraries panel, double-click on the newly created element.
6. Check Edit >> Transparency Blend Space: your asset is in CMYK!
7. Change it to RGB.
8. Save.
9. Close doc.
10. Go to the CC Libraries panel, and open the element again.
11. Check again the Transparency Blend Space: it ignored the change to RGB and remains in CMYK!
I have made some testing and it seems that, as far as you use the RGB element in a RGB document, it will preserve color values and attributions - so, if you know what you doing and how to handle color, not risk whatsoever.
Nevertheless, it is weird that InDesign CC Libraries does not allow RGB space, as it leads to confusion...