Hi Niklas,
The color gradient in this figure can be changed by using the colormap function. Predefined colormaps are listed here, but any custom color map can be used by inputting a m x 3 numeric matrix into colormap. See example below:
>> f = @(x,y,z) x.^2 + y.^2 - z.^2;
>> fimplicit3(f)
>> colormap("cool")
Hope this helps!