Reducing colors in photoshop? (making many colours fewer)

Question

Hi! I've been working on some pictures that will be printed on T-shirts. The problem is that I've got alot of different color in the pictures which would result in a very expensive print. What I would want to be able to do is to reduce the colors in the pictures to only 4 specific colors (plus black and white). The colors must be very specific. for an example:  "ff2600". 
To be totally clear: I need the ALL the different types of yellow into be transformed to just one type of yellow.
So my question is am I able to do something like this in photoshop?
I'd be very grateful for some answers!
Thank you!
// Hannes

Answer 1

Posterize would not work, since it acts on each color channel independantly. You wouldn't have to save as a GIF, though. Just change the mode to Indexed Color.
Use the "Custom" palette, with Colors set to 6 (your four inks plus black and white), and "Forced" set to "Black and White". You can edit your palette as desired, and save it for later use. It can be tricky to get the image to look natural doing this, though; you may be better off starting with something like "Local Perceptual", and then switching to Custom to tweak it.
Once you have the image looking the way you want, you can use the Magic Wand to select each of the colors, create new channels, and fill the selected area in them with black. You can then use these alpha channels to make your screens.