Just curious how this is done. I'm pretty new to both these tools.
I have a color photo that i would like to make black and white. except for 1 color. lets say red. in lightroom (preferably) or photoshop how could i accomplish this?
Are you interested in spot colour (i.e. only one thing in the image is in colour) or are you interested in having all of the reds in the entire image show up?
I'm only familiar with the former (though I'm sure the latter could be acomplished with selective desaturation), so here's an easy way to do spot colouring. Since you have a colour photo to start with, it's fairly easy (in PS...never tried in it LR):
1. Convert to B&W using a layer-based method; in PS3 you have some nice presets in the Channel Mixer. Whatever you do, do NOT simply convert the mode to greyscale.
2. Now expose the colour in the BG layer by "painting" away the layer mask on the Channel Mixer layer (which you created in step 1) to expose the colours underneath. Anything you paint black with show through, anything with white will not. Shades of grey will allow progressively less or more of the BG colour.
It's a great technique, if used appropriately (i.e. not too often).