Been trying to work out a two-colour slip stitch heart design. Worked one out a while ago but it didn't come out correctly. Finally had another go yesterday and was successful. You have to design the card on paper, and then invert the colours on every other row, and use height elongation on the knitting machine. It's also quicker if you have a colour changer.
Two-colour slip stitch (sometimes called maze or mosaic) is useful because it produces a fabric with very short floats on the reverse side. Useful for garments where fingers or toes might get caught