When the same colour isn't
Specify your accent colour as a Pantone spot colour on the invitation and as an RGB hex value on the digital proof you email to the printer, and you have just ordered two different things. The printer reads each specification independently. If neither is explicitly bridged to the other, they may well produce two plates — one mixed from the Pantone formula book, one converted from screen values through CMYK — and you will receive two invoices and, very possibly, two slightly different shades of what you thought was one colour.
This is not a printer error. It is a naming problem.
Colour systems are not interchangeable. Pantone (formally the Pantone Matching System, or PMS) is a proprietary library of pre-mixed inks, each identified by a number and suffix — 485 C for a coated stock, 485 U for uncoated, and these are not identical results despite the shared number. CMYK is a four-ink printing process that mixes cyan, magenta, yellow and black on press to approximate colours from a much narrower gamut. RGB is a screen model that has no direct print equivalent and must be converted before any ink is specified. None of these is a synonym for another, and a conversion between them is always an approximation.
The practical consequence: wherever your stationery suite touches a printer — invitation, envelope liner, menu card, order of service, place card, signage — the colour must be named in exactly one system, in exactly one way, and that specification must be identical across every file. If you use Pantone 7426 C on the invite artwork, the menu card file must also say Pantone 7426 C, not "dusty rose" in a text note, not #B5566A in the RGB field, not a CMYK build that someone eyeballed as close enough.
Pick one system at the start. If the job is entirely offset-printed on coated stock, Pantone is the most reliable anchor. If it is entirely digital (inkjet or laser), specify CMYK and convert from there. Get the artwork file built around that choice, and never let a second name for the same colour appear anywhere in the file set.
One colour, one name, once.