a vast trove of literature on sensory testing regarding sugar and other
sweeteners, as well as testing relative sweetness. For example,
the concentration of a sugar in solution can affect the perception
of sweetness - this is particularly true for the invert sugars,
glucose and fructose, and for the sugar alcohol, xylitol.
Temperature is also a factor in sweetness perception.
Color has a strong
effect on perceived sweetness - red makes people think something is
sweeter than any other color; and green and yellow (associated with lemon and
lime flavors) gives the perception of less sweetness, even when the
colored solutions are all the same concentration. For this reason,
sensory testing is often done in rooms with red or green light, which
masks color differences in the foods being tested.
Brown sugar is an interesting case. Although brown sugar has a lower amount of sucrose than refined sugar (92-96% depending on the manufacturer, vs. 99+%), it can be perceived as sweeter than refined sugar. This is because of the presence of ash components, invert, and sugar degradation/browning products that enhance sweet perception. Brown sugar is a complex mixture of ash, invert, small molecules with a sweet / caramel / brown sugar taste, acetic acid, and coloring polymers.
High molecular weight coloring polymers (Maillard products and browning polymers) are perceived as bitter, so with brown sugar, you have several tastes in the mix. Since each manufacturer has his own way of making brown sugar, one can only make very general statements about it.