Should I design in RGB or CMYK?

We print on Epson 6-tank PBCMYKG dye-based printers, and our print process works from RGB. A CMYK file is converted back to RGB before it prints, so converting it yourself adds an extra step where colours can shift.

Some neon RGB tones (bright greens, electric cyans, vivid purples) are brighter than any printer can reproduce and will print a little duller than on screen. If you make your file in Illustrator, export it with the High Quality Print preset, as Press Quality and PDF/X-1a convert everything to CMYK.