You can use mental ray for Maya’s detail shadow
map to render colored shadows in your scene. This effect is driven
by the transparency color of the object, like raytraced shadows.
To
render colored shadows with detail shadow maps
In
the Scene View, select the light that you want to use to create
a shadow. The light must be a point light, spot light or a directional
light.
Open
the Attribute Editor for the light
shape node.
In
the Attribute Editor, open the Shadows section,
and set the Shadow Color.
Expand
the mental ray section and select
the Detail Shadow Map option under
the Shadow Map Format attribute
in the Shadows section.
Select
the Use mental ray shadow map overrides option.