color start = color(255, 0, 0); // outermost color
color end = color(0, 0, 255); // innermost color

float complexity = 10; // number of steps between colors
float finalDiameter = 300; // size of outermost circle

size(500,500);
background(0);

// for each shade...
for( int i = 0; i < complexity; i++ ) {
  // calculate a decimal from 0 to 1 - this number will
  // increase incrementally to represent progress
  float ratio = i/complexity;

  // create a new color with the output from the lerpColor
  // interpolating function
  // we're using "ratio" to determine where along the
  // interpolated scale to look  
  color currentShade = lerpColor(start, end, ratio);
  
  // then fill with that color
  fill(currentShade);
  
  // this next bit figures out a slightly smaller diameter each time
  float currentDiameter = finalDiameter-(ratio*finalDiameter);
  
  // and draws an ellipse with it
  ellipse(width/2, height/2, currentDiameter, currentDiameter);
}