Is there another approach to get a continuous rainbow of colours around
the perimeter of the circle, and then do a radial blend from white in
the centre to the perimeter?
If it doesn't have to be vector then do it in GIMP. On your canvas, create
a circle select and fill it with the "Full saturation sectrum" gradient
using the shape, "conical (asym)". Then select a white to transparent
gradient and fill the same circle with the "radial" shape selected. With
the circle still selected, transform (or scale) it so it's an oval. Huzaah!
Well, I got my ellipse with rainbow perimeter converging to white at
the centre just great!
I want this ellipse to be the flat top of an inverted cone, with the
colours on the visible "side" of the cone converging to black at the
point, similar to the HSV inverted cone in <http://en.wikipedia.org/wiki/Image:HSV_cone.png>,
but without all the labels and arrows.
Extrapolating your directions for the ellipse above, I created a
rainbow gradient from one side of the ellipse to the other just fine.
I created a square that has the left edge green and transforms through
a number of colours to purple on the right edge.
Then I tried to squeeze the black base of the square to a point halfway
from left to right on the bottom edge using the Perspective (keystone)
tool. After reading section 4.8, 'Perspective", I select the entire
graphic, double-click the Perspective tool, set
@ Direction (.) Normal
@ Interpolation: [Cubic ]
@ Clipping: [Crop to result ]
@ Preview: [Image + Grid ]
@ [ Number of grid lines ] 
Then I click on the graphic and move the bottom L handle to the middle
of the bottom edge, and the bottom R handle also to the middle of the
bottom edge so that there is a very short edge. The horizontal grid
lines are parallel with the bottom edge. Then I click the Transform
button and ... nothing happens.
So how to get the graphic to narrow from top to bottom?
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