How can I put random number values into "rgb()" for the svg "fill" attribute?

I have an svg element on my website, and I want to animate it so that it changes from a random color value to another random color value. What I have right now looks like this:

<?xml version="1.0" standalone="no"?>
<svg width="8cm" height="3cm" viewBox="0 0 800 300" xmlns="http://www.w3.org/2000/svg">
  <desc>Example anim01 - demonstrate animation elements</desc>
  <rect x="1" y="1" width="900" height="400"
        fill="none" stroke="none" stroke-width="2" />
  <!-- The following illustrates the use of the 'animate' element
        to animate a rectangles x, y, and width attributes so that
        the rectangle grows to ultimately fill the viewport. -->
  <rect id="RectElement" x="300" y="100" width="300" height="100"
        fill="rgb(0,255,0)"  >
    <animate attributeName="fill" begin="0" dur="1.3"
    fill="remove" from="rgb(0,255,0)" to="rgb(255,0,0)" repeatCount="indefinite"/>
    <animate attributeName="x" begin="0s" dur="1.3s"
             fill="freeze" from="300" to="0" repeatCount="indefinite"/>
    <animate attributeName="y" begin="0s" dur="1.3s"
             fill="freeze" from="100" to="0" repeatCount="indefinite"/>
    <animate attributeName="width" begin="0s" dur="1.3s"
             fill="freeze" from="300" to="800" repeatCount="indefinite"/>
    <animate attributeName="height" begin="0s" dur="1.3s"
             fill="freeze" from="100" to="300" repeatCount="indefinite"/>
    
    
  </rect>
  <!-- Set up a new user coordinate system so that
        the text string's origin is at (0,0), allowing
        rotation and scale relative to the new origin -->
  <g transform="translate(100,100)" >
    <!-- The following illustrates the use of the 'set', 'animateMotion',
         'animate' and 'animateTransform' elements. The 'text' element
         below starts off hidden (i.e., invisible). At 3 seconds, it:
           * becomes visible
           * continuously moves diagonally across the viewport
           * changes color from blue to dark red
           * rotates from -30 to zero degrees
           * scales by a factor of three. -->
    <text id="TextElement" x="0" y="0"
          font-family="Verdana" font-size="35.27" visibility="hidden"  >
      Buy our crap!
      <set attributeName="visibility" to="visible"
           begin="0s" dur="1.3s" fill="freeze" repeatCount="indefinite"/>
      <animateMotion path="M 0 0 L 100 100"
           begin="0s" dur="1.3s" fill="freeze" repeatCount="indefinite"/>
      
      <animateTransform attributeName="transform"
           type="rotate" from="-30" to="0"
           begin="0s" dur="1.3s" fill="freeze" repeatCount="indefinite"/>
      <animateTransform attributeName="transform"
           type="scale" from="0.5" to="2" additive="sum"
           begin="0s" dur="1.3s" fill="freeze" repeatCount="indefinite"/>
    </text>
  </g>
</svg>

But what I want is instead for the rectangle to be assigned a random color value (something like "rgb(random#,random#,random#)")

How would I go about doing that to make sure that every single color is possible?