Which is interesting because I am printing digital photos which I edit on an RGB screen.

I paid for college in part by doing digital prepress. We had CMYK and 8 and 12 color separations.

CMYK always has a dramatic color shift from any on-screen colorspace. Vivid green is really hard to get. Neons are (kinda obviously) impossible. And, hilariously/ironically (given how prevalent they are), all manor of skin tones are tough too.

Photoshop and Illustrator let you work in CMYK, and is directionally correct. Ask your printer if they accept those natively.