Simple request - should be simple, right?
1. Flatten image
2. Convert to grayscale
And, you're done....
but...
Watch for adjacent colours that change to the same shade of grey, or coloured lettering that has lost it's contrast with its background.
So, you may have to go back into your colour original and do some colour nudging/tweaking, or changing, if necessary, and repeat the above process.
A slight change in the above steps can lead to better results, sometimes, depending on the "offending" colours. Instead of just hitting the "convert to grayscale" mode command, try this instead:
1. Mode: convert to Lab colour
2: Delete the a + b layers, leaving only the "l" or "lightness" channel
3. Convert to grayscale
4. Adjust your layers for proper "colour break"
5. Flatten
I'm sure there are other ways, but this should help.
Convert to grayscale uses a different technique to represent colours-as-gray than the second technique, your mileage may vary, but the lightness channel in an LAB colourspace does not, for example, convert all dark reds to black as the grayscale conversion will. I tend to use the second technique more often, but I'm curious to see what other techniques are out there...
dmz