triangles.js

var TRIANGLE_WIDTH, TRIANGLE_HEIGHT, NUM_ROWS, TRIANGLE_WIDTH_HALF;

function setup() {
  createCanvas(750, 750);
  // background('#f9a825');

  NUM_ROWS = 8; //Modify me
  TRIANGLE_WIDTH = height / NUM_ROWS;
  TRIANGLE_WIDTH_HALF = (TRIANGLE_WIDTH / 2);
  TRIANGLE_HEIGHT =  TRIANGLE_WIDTH;
}

function draw() {
  var alt = true;
  var currentRow = 0;

  // var currentRowInverse = 0;
  // var altInverse = true;

  var y2Inverse = height; //y2 value for inverse triangle

  for (var y2 = height - TRIANGLE_HEIGHT; y2 >= 0; y1Inverse -= TRIANGLE_HEIGHT, y1 -= TRIANGLE_HEIGHT, y2Inverse -= TRIANGLE_HEIGHT, y2 -= TRIANGLE_HEIGHT, y3Inverse -= TRIANGLE_HEIGHT, y3 -= TRIANGLE_HEIGHT) {
    currentRow += 1; //Counts rows
    alt = !alt; //Alternates certain values

    var y1 = y2 + TRIANGLE_HEIGHT; //y1 value for normal triangle
    var y1Inverse = y2Inverse - TRIANGLE_HEIGHT; //y1 value for inverse triangle

    var y3 = y2 + TRIANGLE_HEIGHT; //y3 value for normal triangle
    var y3Inverse = y2Inverse - TRIANGLE_HEIGHT; //y3 value for inverse triangle

    //Math crap to determine color of normal triangle
    var r = 0, g = 0, b = 0;

    r = (r + (currentRow == 4 ? 121 : 30 * currentRow));

    g = (g + (13 * currentRow));

    b = (b + (currentRow == 4 ? 45 : 11 * currentRow));

    var rowColor = color(38 + r, 77 + g, 115 - b); //set color of row based on above r g b

    stroke(rowColor);
    fill(rowColor);

    var x2Inverse = alt ? 0 : TRIANGLE_WIDTH_HALF;

    //Determines x1, x2, x3 values for normal triangle
    for (var x2 = alt ? TRIANGLE_WIDTH_HALF : 0; x2 <= width; x1 += TRIANGLE_WIDTH, x2 += TRIANGLE_WIDTH, x3 += TRIANGLE_WIDTH) {

      var x1 = x2 - TRIANGLE_WIDTH_HALF;
      var x3 = x2 + TRIANGLE_WIDTH_HALF;

      triangle(x1, y1, x2, y2, x3, y3);
    }

    //More moath crap to determine color of inverse triangle
    var rInverse = 0, gInverse = 0, bInverse = 0;

    rInverse = (rInverse + (currentRow == 4 ? 121 : 30 * currentRow));

    gInverse = (gInverse + (13 * currentRow));

    bInverse = (bInverse + (currentRow == 4 ? 45 : 11 * currentRow));

    //set color of row based on above r g b
    var rowColorInverse = color(98 + rInverse, 103 + gInverse, 93 - bInverse);

    stroke(rowColorInverse);
    fill(rowColorInverse);

    //Determines x1, x2, x3 values for inverse triangle
    for (var x2Inverse = alt ? 0 : TRIANGLE_WIDTH_HALF; x2Inverse <= width; x1Inverse += TRIANGLE_WIDTH, x2Inverse += TRIANGLE_WIDTH, x3Inverse += TRIANGLE_WIDTH) {

      var x1Inverse = x2Inverse - TRIANGLE_WIDTH_HALF;
      var x3Inverse = x2Inverse + TRIANGLE_WIDTH_HALF;

      triangle(x1Inverse, y1Inverse, x2Inverse, y2Inverse, x3Inverse, y3Inverse);
    }
  }
}

Comments:

• in the for loop head (for (etc)), you don't need to increment x1 and x3 (or x1Inverse and x3Inverse) because you're making new ones in every loop. (var x1 = ... etc), so you can chop those statements
• you can combine making the two triangles in the same loop, if you change the for loop to a while loop, and manually increment x2 and x2Inverse at the bottom of the while loop