File content as of revision 2:381421ec57e1:

#include "mbed.h"
#include "WS2812.h"
#include "PixelArray.h"

#define WS2812_BUF 77   //number of LEDs in the array
#define NUM_COLORS 6    //number of colors to store in the array
#define NUM_STEPS 8    //number of steps between colors

DigitalIn usrBtn(USER_BUTTON);
DigitalOut usrLed(LED1);
PixelArray px(WS2812_BUF);

// See the program page for information on the timing numbers
WS2812 ws(D9, WS2812_BUF, 6,17,9,14);   //nucleo-f411re

int color_set(uint8_t red,uint8_t green, uint8_t blue)
{
  return ((red<<16) + (green<<8) + blue);   
}

// 0 <= stepNumber <= lastStepNumber
int interpolate(int startValue, int endValue, int stepNumber, int lastStepNumber)
{
    return (endValue - startValue) * stepNumber / lastStepNumber + startValue;
}

int main()
{
    int colorIdx = 0;
    int colorTo = 0;
    int colorFrom = 0;
    
    uint8_t ir = 0;
    uint8_t ig = 0;
    uint8_t ib = 0;
    
    ws.useII(WS2812::PER_PIXEL); // use per-pixel intensity scaling
    
    // set up the colours we want to draw with
    int colorbuf[NUM_COLORS] = {0x2f0000,0x2f2f00,0x002f00,0x002f2f,0x00002f,0x2f002f};
    
    // Now the buffer is written, write it to the led array.
    while (1) 
    {
        //get starting RGB components for interpolation
        std::size_t c1 = colorbuf[colorFrom];
        std::size_t r1 = (c1 & 0xff0000) >> 16;
        std::size_t g1 = (c1 & 0x00ff00) >> 8;
        std::size_t b1 = (c1 & 0x0000ff);
        
        printf("Starting colors (r,g,b): %d %d %d\n",r1,g1,b1);
        
        //get ending RGB components for interpolation
        std::size_t c2 = colorbuf[colorTo];
        std::size_t r2 = (c2 & 0xff0000) >> 16;
        std::size_t g2 = (c2 & 0x00ff00) >> 8;
        std::size_t b2 = (c2 & 0x0000ff);
        
        printf("Ending colors (r,g,b): %d %d %d\n",r2,g2,b2);
        
        for (int i = 0; i <= NUM_STEPS; i++)
        {
            ir = interpolate(r1, r2, i, NUM_STEPS);
            ig = interpolate(g1, g2, i, NUM_STEPS);
            ib = interpolate(b1, b2, i, NUM_STEPS);
            
            //write the color value for each pixel
            px.SetAll(color_set(ir,ig,ib));
            
            //write the II value for each pixel
            px.SetAllI(32);
            
            for (int i = WS2812_BUF; i >= 0; i--) 
            {
                ws.write(px.getBuf());
            }
        }
        
        colorFrom = colorIdx;
        colorIdx++;
        
        if (colorIdx >= NUM_COLORS)
        {
            colorIdx = 0;
        }
        
        colorTo = colorIdx;
    }
}