The good news is that Pantone Blue (assuming you mean standard 072C) has fairly reliable CMYK and RGB gamut recipes:
R28 G63 B128 or C100 M88 K5
The bad news?
Generating Pantone equivalent solid colors from an RGB workflow that doesn't support any kind of industry standard color profiling is going to be *complete* voodoo.
I don't know enough about Gimp to be helpful with the specific levers and buttons necessary to make it perform said magic, but unless you can specify exact color proportions in CMYK (which is what the printer is going to use for process color) then you would need a device-specific calibration profile (typically an ICC profile) from your printer to make the necessary RGB to CMYK gamut conversion. If that's not an option, then the only way to get a reliable representation of the blue from them is going to be the "throwing darts" method because every piece of software gets from RGB to CMYK via a conversion table and there are many - so you need control over which one is used.
Literally until the past few years, color management was the bane of most designers' existences. Printing the same job with 5 different printers or on 5 different devices, until very recently, would always give you 5 measurably different results.
I'll happily help you with the file, as with the right tools it's a trivial step - but without them it's a complete pain in the pesqueeter.
no subject
The good news is that Pantone Blue (assuming you mean standard 072C) has fairly reliable CMYK and RGB gamut recipes:
R28 G63 B128
or
C100 M88 K5
The bad news?
Generating Pantone equivalent solid colors from an RGB workflow that doesn't support any kind of industry standard color profiling is going to be *complete* voodoo.
I don't know enough about Gimp to be helpful with the specific levers and buttons necessary to make it perform said magic, but unless you can specify exact color proportions in CMYK (which is what the printer is going to use for process color) then you would need a device-specific calibration profile (typically an ICC profile) from your printer to make the necessary RGB to CMYK gamut conversion. If that's not an option, then the only way to get a reliable representation of the blue from them is going to be the "throwing darts" method because every piece of software gets from RGB to CMYK via a conversion table and there are many - so you need control over which one is used.
Literally until the past few years, color management was the bane of most designers' existences. Printing the same job with 5 different printers or on 5 different devices, until very recently, would always give you 5 measurably different results.
I'll happily help you with the file, as with the right tools it's a trivial step - but without them it's a complete pain in the pesqueeter.