Mercury is one of the Three Alchemical Principles and is aligned with Spirit. The Mercury-headed Wand is an alternative design to the Pentagram-headed Wand used by the Chief Adept in the Portal Hall (The Second Degree).
When ordering the Mercury Wand as one in a set of Three Alchemical Wands, you have three color options: 1) unpainted, 2) yellow, and 3) blue.
Option 1 (unpainted) will allow you to paint the wand any way you like.
Option 2 (yellow) stresses the Mercury Wand as having an affinity with Air, the reconciling element between Fire and Water or between the red Sulphur Wand and the blue Salt Wand. The yellow color of Mercury is a natural choice for the Mercury Wand as part of the Trio of Wands used in a Golden Dawn Portal Ceremony.
Option 3 (blue) stresses alchemical Mercury as having an affinity with Water. As part of a trio of Alchemical Wands, the blue Mercury Wand would be accompanied by the red Sulphur Wand and the yellow Salt Wand. (In Alchemy, Mercury and Sulphur represent Spirit and Soul or Water and Fire, with Salt representing the reconciling Body that contains Mercury and Sulphur. In this scenario, Salt has an affinity with the reconciling element of Air.)