changing the color of shadows on a material?

so its not just black?

place another lamp at different angle and make it the color you want the shadow to be

The old Blender docs go into the basics of the “three lamp” setup which gives an introduction to playing with light and shadow.

I think there are about 5 different methods to get coloured shadows (and some more if I will think longer). The problem is not the coloured shadow, but to keep the rest of the objects in a neutral color.

  1. Two lightsources with complementary colors (Yellow/Blue, Cyan/Red and the like).
  2. Two lightsources (same place, same strength), first light no shadow, second light with complementary colour with shadow.
  3. Three lightsources, - first light no shadow, second light negative and shadow, third light lightens the objects.
  4. Colour filters (coloured glas, set the “Filt” Value)
  5. Ramp shader set to energy (this is an easy and good method).
  6. Ambient light, play with the “Amb” Value of the materials.

There are obviously more methods than these.

I think the easiest way to do it is to render in two passes. First render with shadows turned off on the lamps. Then render with shadows on but make all the objects shadow only. Also render the image with an alpha channel. Then in a compositing program, possibly the Blender sequencer, composite the shadows on top but adjust the colour.

You can use this method to blur raytraced shadows too.

But Blender having coloured shadows would make it much easier.

you can also tint stuff with negative lamp (at laeast in older blenders) set to inverse color of desired one.