Before you go off half cocked. Find out what kind of glass you have. Some store front glass will BREAK when film is applied. My ex boss decided to put film over dark windows. West facing in Phoenix. The offices on the west side of the building approached 95 degrees in the summer time. Every contractor he called said the same thing put film on the windows at your risk. NO guarntee on window breakage. Pretty expensive if they break.
The lighter colors will block 10-20% of the visiable light and are worthless for heat gain. (My opinion) Cloudy days you will see a decrease of natural light.
Your sign will be dimmer. Less noticeable.
That will depend day to day as the sun moves. Awning sounds like it might be the hot ticket here.
I suggest that you contact pros in your area and ask them for a bid for doing the work. Permission from the building owner in writing and advance. There may be issues that you have not thought of.
Interior solar shades with your stores name and/or WE'RE OPEN on them might be a better solution. They can be raised in the afternoon and the winter time will not be dark.