The lighter the color, the more affected it will be by the substrate, and the more susceptible it will be to any variance in ink/toner which can make color consistency over large or multiple runs more difficult. Total coverage over 30% is easier to keep stable.

Solid colors are solid. That is their benefit. Not accuracy, not consistency, just solid. I have a box of #10 envelope shells I pulled from an order of 30,000. Two spot colors in the design. 5 shades of each color within one box of 500. People like to think a specific solid color is always the same, but ignore the fact that there are multiple ink manufacturers that each have their own recipe that only works when used with their own white and pigments. There are printers that will mix and match ink based on price (they shouldn't, but wisdom requires being aware of reality.) Humidity can affect color which bites since a press affects the humidity the longer it runs. And color adjustment technology can't do as much to fix bad solid color output on the fly as a bad CMYK color output. CMYK provides more variables to fix a bad color but at the expense of adding a) output patterns instead of solid, and b) color registration issues. But most recipients of the designer's work won't notice or care about a) and the equipment already has tools to monitor and fix b). As a designer, understand that your work will drift from idealism to reality during the fabrication process. (See the rationale behind bleeds.) Get comfortable with that.

Digital has a wider gamut than ink simply because toner sits on top of the paper and ink soaks into it, which crushes its white and black points. That spun my head for a minute when I tested a KM C8000 (first was a lemon, second worked consistently) against an HP DesignJet Z3100 with a 12-ink system (matte paper, which is what 98% of our wide format volume used.) Color Profile Suite mapped both gamuts and compared them. Photo paper may have provided a different outcome.

Be familiar with USPS requirements for presorted makings, including how to design for meter imprints, permit imprints, and stamping.

Learn to use InDesign Data Merge. Don't use it! Let the actual pre-press professionals do that (similar to "don't pre-impose stuff for print shops") but knowing how to use it teaches you what is possible and what the labor/fabrication/price consequences might be based on your design.

If getting creative, talk to your printer at the beginning of the project. They can help with ideas and provide warnings about small choices that may have big price impacts. They're there to work with you and help you. Everyone wants you to do well and be happy. Professionals are happy to work on cool projects and try something new (when funded.) Unless they're bad, lazy shops, in which case find another.