In 4.3, a light blue color for non-printable characters are established, which was a huge improvement in usability for Writer. 
Following the same logic as the guide and grid, are non-printable elements that can be benefited by this improvement in Impress and Draw.
Although the color can be changed manually, set default values, ensures better user experience.

Proposal:
Grid: Light blue or blue solarized
Guide: Ligth yellow or yellow solarized

Thanks