This SVG example illustrates how to color an ellipse with a radial color gradient.

RadialGradient.svg

<svg width="600" height="400" xmlns="http://www.w3.org/2000/svg">
  <defs>
    <radialGradient id="idRadGrad" cx="50%" cy="50%" r="50%" fr="0%" fx="50%" fy="50%">
      <stop offset="0%" style="stop-color:rgb(255,0,0); stop-opacity:0" />
      <stop offset="100%" style="stop-color:rgb(0,0,0); stop-opacity:1" />
    </radialGradient>
  </defs>
  <ellipse cx="300" cy="200" rx="275" ry="175" fill="url(#idRadGrad)" />
</svg>

Output