RGB and CMYK are colour formats that are used in 2 different fields. RGB CMYK are commonly referred to as ‘screen’ and ‘print’.
RGB is used for screen devices such as tablets, monitors, TVs and phones and uses the 3 colours (Red, Green, Blue) to create all the colours needed. White is created by combining all the colours.
CMYK is used in the printing industry and uses the 4 inks (Cyan, Magenta, Yellow, Black) to create the colours needed for printing materials. White is obtained by using the paper colour and having no ink printed on the areas that need to be white.
Until recently when you wanted to design a brochure you would need to use CMYK formatted images, typically all devices that would take these images would create them in an RGB format. The designer or printer would then need to convert the image files to RGB by using software such as Photoshop. During this process, you would lose a little vibrancy due to the CMYK format being optimised for print and not screen (blues would see the most noticeable change and tend to look flatter).
Due to this, the colour you see on screen isn’t necessarily the colours you are going to see in print, as they are 2 totally different colour systems. So it is imperative that you see a printed proof before proceeding to print to check you are happy with the colours.
Advances in software recently have helped and in some cases you will see better results if you supply images in RBG, saving the designer or printer from having to convert these. High-end brochures and photography will still rely on the designer/photographer to convert image files from one format to the other with specialist software and their expert eye.
In short no, any good designer will check image formats and convert these to obtain maximum quality, if this is factored into the project cost. Some will advise if there is a problem but some will just reproduce what you supply, this is when you need to check the images in a physical proof before signing off for print. Once you have signed off the proof the printed material is your responsibility.