I had a problem (sadly, not the only one on this particular print) with the colours for the upholstery on the tube train seats. It’s so difficult to predict what will work best with the surrounding areas and once the blocks are cut, it’s very hard to rectify.
In the end, I printed up little sections with various combinations of the four colours I was using and laid them down over one of the earlier completed proofs.
Here are three examples:

Once I’d decided on the colour combination to use, I had to take the one block that already had the diamond shapes cut into it and offset it on to the other three blocks.
Here’s a very quick explanation of offsetting:
Ink up a block very thickly and print it on paper.
Place an uncut block on the press, lay down the printed paper on top in exactly the same place and run it through the press again.
You then have a perfect impression of the cutting from the original block transferred to the next one. (Apologies if I’m preaching to the converted…)
Below are three random photos of the offsetting as I was going along – the white powder on the first one is talcum powder, which dries the offset ink quickly so you can cut into the block without waiting for the ink to dry.