I am making a design to be printed on a plastic bag. The design is a maroon/magenta (#4c1429) block background with white writing.
From what I understand, RGB is best for things remaining online, and CMYK is best for prints, but usually said prints are on paper. Any direction on the best colour space for plastic?
The design you describe would likely be printed using spot colors. One of the most popular spot color library is called the Pantone Matching System (PMS), if you're using Photoshop or Illustrator then the swatches are built right in to the application. However, there's no guaranteeing that your printer will use that library in house.
The golden rule for these types of questions is always going to be "Ask your printer!". Not every printer will use the same process, and not every process is the same. It's less of a headache to start knowing what color system will be used for print and design using that, but generally, it's safe to start with CMYK or PMS then convert later if necessary.
