A waistcoat (vest) usually cut on the bias in the same tartan as the kilt, but not necessarily the same colour shades, is particularly appropriate, especially in the evening.

Hose knitted on the bias in the same tartan as the kilt is appropriate for evening black or white tie. Rarely will the colour shades exactly match the kilt.

People also wear ties and garter flashes in tartan to match the kilt but this is generally considered undesirable by well-informed kilt wearers.

Tartan sporrans and bonnets should be avoided like the plague!

As others have said, wear the tartan tie when you are in bifurcated garments.