Your perseverance deserves help. Some printers (companies) will list CMYK in their requirements although send a RGB and they use it anyway.
Whichever way you do it there is a change in colours (look up gamut). Some RGB colors that you can see on your monitor (in particular, blue, green and all bright vibrant colors) cannot be printed and/or replicated with standard CMYK inks.
Nothing you can do about that, look for a printer that uses laser printers, those Noritsu, for example, work in RGB and any half decent printer should supply icc's for their paper.
Gimp works in RGB. There is a plugin CYAN that as a final stage will make a cmyk file. (edit: CYAN also works stand-alone)
A bit about it here
https://www.gimp-forum.net/Thread-CMYK- ... -Gimp-2-10 That is a bit old now, worth a look.
Possibly for you, a better way is using the free graphics program Krita. You can open a RGB image, change the colour space and export a CMYK tif or jpeg.
Quote:
..it is impossible to know what a photo on a computer screen will look like on a big format paper print.
Very, very true. The serious users have their monitors color calibrated. I use a laptop, not calibrated but with brightness turned down to get a better representation (I know my computer). Gimp does have a soft-proofing option see:
https://www.gimp-forum.net/Thread-The-R ... t-Proofing but it is not 100% accurate when it comes to out-of-gamut colours.
If you are worried, crop a 6" x 4" area out and get a print of that. Look at colours / brightness / contrast that might need tweaking. Some good printers do that for free. (edit: or send you a sample/calibration print +image)
best of luck