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.
Grid: Light blue or blue solarized
Guide: Ligth yellow or yellow solarized
This reminds me as iWork works, but it's a good idea. Greatly improve the appearance of LibreOffice still very gray (very very gray), some color would look more modern and attractive. I also think visually impaired trying to distinguish between much gray.
Move to close this => WF
bug 117348 has now set grid to the COL_GRAY7 increasing contrast as needed, colors for Grid and Guide as here are not well founded.
We discussed this topic in the design meeting. As stated in comment 3 we changed the default color recently to have more contrast and making it blue would be the opposite. But on the other hand users like customization and in case of a dark background, in particular with a dark mode see bug 118017 and bug 118320, a different color becomes handy. So we recommend to make the grid color an user option (Tools > Options > Application Colors).
My fault, the color option exists right now and it is possible to have white dots on black background. And of course also blue.