MCP:

Take it from a landlord who does more painting than any homeowner:

It is not necessary to match paint colours exactly to have the room look like it was all painted with the same paint. As long as you get a reasonably CLOSE match to the colour, just paint to the closest corners, and stop there.

You see, your eye sees different colours on each side of a corner because of the different lighting and different viewing angles, and so your brain will automatically attribute any small difference in colour across a corner to the different lighting and viewing angles and won't notice any actual difference in the paint colour.

I do this all the time when a tenant causes damage to a wall. Over years, the old paint tends to get dirty just from airborne dirt sticking to the walls. I can't be starting to repaint an entire apartment just because one wall was damaged. So, I simply repaint the one wall up to the closest corners, and you can't tell it was only the one wall that was painted.

Now, obviously you can't have a difference in colour across a corner, like red and green. But, small differences in the tint or tone of a colour will never be noticed because the brain attributes those differences to the different lighting and viewing angles across a corner.

So, just get a swatch that's a reasonably close match, and you should be OK.