k = 0.5;
ListDensityPlot[
Table[k Sin[3 x + y^2], {x, -3, 3, 0.1}, {y, -3, 3, 0.1}],
ColorFunction -> (ColorData["VisibleSpectrum"][
     Rescale[#, k*{-1, 1}, {380, 750}]] &),
ColorFunctionScaling -> False, PlotRange -> {-1, 1}]

p1 = DensityPlot[Sin[x] Sin[y], {x, -3, 3}, {y, -3, 3}, ColorFunction -> (Hue[2/3 (1 - #)] &), PlotLegends -> Automatic, PlotPoints -> 50] p2 = DensityPlot[Abs[Sin[x] Sin[y]], {x, -3, 3}, {y, -3, 3}, ColorFunction -> (Hue[1/3 (1 - #)] &), PlotLegends -> Automatic, PlotPoints -> 50]