Embedding a color in GIMP

[TiaS]

I hope this is the right section to ask this. I am trying out a new printing company and they require photos to be embeded in an assinged color and for the photos to be 300ppi. I sent them 5 photos for a test print. But the issue I am having is frustrating.

-I made all 5 photos into 300 ppi (4 of them were only 72ppi). They said that 2 of my photos were only 200ppi, but my GIMP program said they were all 300ppi.

-The second issue is that one of my photos, they said was not assigned to a color. I went back and converted it to RGB, and sent it is again. They said the same thing. GIMP says that it is RGB, but they say no color is assigned.

I am having them print all my photos anyway, as it is just a test print. Why are only some of my photos meeting their requirments on their end, when, to the best of my knowledge, they all meet it on my end in GIMP?

 

[KmH]

Uh! Not assigned a color - assigned a color space. Color space - Wikipedia, the free encyclopedia

RGB has many color spaces - sRGB, Adobe RGB, Monitor RGB, ProPhoto RGB, scRGB, ROMM RGB, Apple RGB, ISO RGB, etc. and is used to print digital photographs either chromogenically or inkjet.
RGB is an additive color model. Color model - Wikipedia, the free encyclopedia

Chromogenic color print - Wikipedia, the free encyclopedia
Inkjet printer - Wikipedia, the free encyclopedia

Then there are color models other than RGB, like CMYK (a suntractive color model) which is used for off-set printing.

Here is a group of tutorials that you may find helpful: Tutorials on Color Management & Printing

The PPI assigned to an image is meaningless until it will be printed. Then the PPI and the pixel dimensions of the image define the print size.
A 3000 pixel by 2000 pixel image assigned to 100 ppi will be a 30" x 20" print.
A 3000 pixel by 2000 pixel image assigned to 300 ppi will be a 10" x 6.67" print.
Do you follow the math?

Additionally not all standard print sizes have the same aspect ratio. Most DSLR camera image sensors (FF and APS-C) have a 3:2 aspect ratio, yet an 8x10 print has a 5:4 aspect ratio, while wallets and 5x7 prints have a 7:5 aspect ratio.