From Dr Lee's website:
Q. What is the explanation for why minoxidil products can vary in color?
A. There are two reasons for the variations in color of minoxidil solutions. (1) Pure minoxidil crystals are white with a slightly yellowish hue. When the minoxidil crystals are milled, i.e. micronized, trace elements can be introduced which result in a change in hue for minoxidil solutions. The color change in no way affects either the efficacy or the safety of the minoxidil compounds. As an analogy, consider the diamond. Not all diamonds are colorless. Diamonds can be blue or yellow or pink, etc. Again, the color is due to the presence of trace elements, but there is no significant change in the chemical or physical composition of the gem. (2) With passing time, all minoxidil solutions become darker, approaching a deep amber color. Even though minoxidil is essentially insoluble in water, the addition of water in the formulation of a minoxidil solution helps to stabilize the solution and to preclude precipitation of the minoxidil. With the passage of time, the hydration of the minoxidil molecule in solution form changes the refractory characteristics, resulting in a yellowish cast that can intensify rapidly, especially with increases in the ambient temperature. The color change in no way affects either the efficacy or the safety of the minoxidil compounds. There is an analogous example in the color of water when it freezes. The change in the physical characteristics makes a
difference in the way the water refracts light. As rain, it is clear. As snow, it is white. But there is no change in the chemical characteristics. It is still H2O.