[ (( [ID] (x1 x2 ... xN) v r g b a v r g b a ... )
((x1 ... xN) v r g b a v r g b a ...) ...)] )
Specifies a function, applied to each N-D vertex, which determines the
colors of N-dimensional objects as shown in camera CAMID.
Each coloring function is defined by a vector (in ID's coordinate system)
[x1 ... xN] and by a sequence of value (v)/color(r g b a) tuples,
ordered by increasing v. The inner product v = P.[x] is linearly
interpolated in this table to give a color.
If ID is omitted, the (xi) vector is assumed in universe coordinates.
The ND-color command specifies a list of such functions; each vertex
is colored by their sum (so e.g. green intensity could indicate
projection along one axis while red indicated another.
An empty list, as in (ND-color CAMID ()), suppresses coloring.
With no second argument, (ND-color CAMID) returns that camera's
color-function list.
Even when coloring is enabled, objects tagged with the keepcolor
appearance attribute are shown in their natural colors.