I mean, you can just do it manually -- with different screens of the same pantone swatch, but it might be a pain if it's a complex image. I'm guessing that it is, since you're asking for "fast".

Yeah, I use to make all my vectors monochromatic green on an online comic I used to create and what I had to do is what EC said and do it manually with one pantone color and using transparency to get different shades, etc.

The only other way I know to do it is to select colors that are the same with the Select>Same fill/stroke color and then change the colors that way. It will be faster, but not by much if you have a complicated image.

as suggest by broacher elsewhere, turn it greyscale, group all your items, copy and paste behind and use pathfinder to turn into one solid shape (black arrow > make compound). then expand. colour the solid shape you just created with the spot you want shades of, then select both and click the black arrow in the corner of transparency and click opacity mask. make sure the solid colour is behind the grouped image of your vector drawing.

That technique works amazingly well, I had never tried that before and was skeptical, but you need to make sure your settings are like this in the transparency tab:



or it will make the image a negative image (at least it did with me), but these settings fix that.

two simple methods:

One is to convert to garayscale first and then with the image selected select SCREEN in your transparency palette and leave it at 100%. THEN place the color you want as your monochrome as a solid square BEHIND the image and leave it in NORMAL mode but play with transparency values until it looks good.

The other method is the same, but you leave the photo alone. This gives you a few tones of the original photo showing through. You can also place another square of color OVER the image and choose COLOR for that one in the transparency pallet and select say 70% or so...again play with it until it looks good. The top square of color will oversatiurate the image (has a nice effect)and overpower the color in the photo so that you don't even have to convert it to grayscale and therefor don't compromise the original integrity of the image.

Also, for fun, you can play with complimentary or competing colors for some cool effects.