Every image has a mode, you can find the mode of any image on Photoshop by going to image then to mode, this is also how you change the mode.
Also you can set the colour mode when you are opening a new document, the option is on there.
The default mode for Photoshop is RGB. If you change the mode to CMYK it will lose some of the options available, this is because Photoshops settings were made with RGB in mind.
Many of the RGB colours sit out side the range of the CMYK palette therefore the colour may shift into a different colour, although it is the closest colour to it.
When you save your image make sure you save it in CMYK mode.
For us to work with consistent colour, you need to work with a swatch palette. Photoshop is very similar to Illustrator in this way, there is a swatch palette with all the default swatches in it. To delete swatches on Photoshop, you need to click alt and click the swatch you want to get rid of.
Using spot colours in Photoshop;
when you are choosing your colour in the colour picker, there is a colour library which you should click on.
Creating a duotone option means that both colours will be printed as spot colours, they will be overlayed in the printing process.
Allows you to map your spot ink to the origional shade of grey in the origional image. You can do this by clicking on the diagonal line and adapting it.
The channel itself looks like this, as it is representing the spot colour in greyscale.]