APA102 + Pico (problem on write data)

It's probably that the data is getting sent out too slowly so the lights think its the start of a frame - in Espruino you can just supply the whole array of data at once, like this:

SPI2.setup({mosi:B15, sck:B13, baud:4000000});
var NumLEDs = 150;
var leds = new Uint8Array(8 + NumLEDs*4);
function draw() {
  var millis = getTime();
  var r = Math.random()*255;
  var g = Math.random()*255;
  var b = Math.random()*255;
  for (var pixel = 4; pixel < leds.length-4; pixel+=4)
  {
    leds[pixel+0] = 0xE0 | 5;
    leds[pixel+1] = b;
    leds[pixel+2] = g;
    leds[pixel+3] = r;
  }
  write_leds(leds, NumLEDs);
}

function write_leds(leds, numLeds) {
  // set start frame
  leds[0] = 0;
  leds[1] = 0;
  leds[2] = 0;
  leds[3] = 0;
  // set end frame
  leds[leds.length-4] = 0xff;
  leds[leds.length-3] = 0xff;
  leds[leds.length-2] = 0xff;
  leds[leds.length-1] = 0xff;
  // Send the data
  SPI2.write(leds);
} // write_leds()

function onInit() {
  setInterval(draw, 50);
}
onInit();

You can use ArrayBufferViews so that you can get a 0-based index for the LEDs, but for now it's probably simpler to just use the single array, and start your LED indices from 4.