I hardly ever use straight pigments for inpainting.
Cadmium orange is one of the most useful colors for inpainting photographs. Most natural materials yellow over time, a little cadmium orange goes a long way to provide some of these tones in "straight" pigments, like ivory black and titanium white. How white is white? Not very usually.
My favorite watercolors are Golden Qor Colors. Why I like them -
Excellent ageing properties
Transparency about formulation, properties, lightfastness
The resin used to create these colors is Aquazol® (water soluble polyoxazoline compounds), not the traditional gum arabic. Why is that important? Aquazol remains soluble in water and ethanol. In contrast, gum arabic is only soluble in water, and becomes slightly less soluble over time. For interventions - such as inpainting, where pigments are added to an artwork in an area of loss - solubility is important.
Conservation ethics require that anything that is added can easily be removed. In other words, it is reversible. So generally, the more options for solvents, the better. This is one reason that the field has moved toward synthetic varnishes and adhesives, and away from natural resins.
Water is a great solvent (and non-toxic too!) but sometimes you need other options. Gelatin and paper layers on photographs, and most organic materials, will swell with water. Ethanol, on the other hand, does not cause the same response, allowing removal or adjustment without swelling the underlying layers. It really feels magical sometimes.