Are you referring to the MSAFluidDemo ? 

Ya I am talking about MSAFluidDemo application only.Its default background is black n i want to change it to some  other color but not getting how to do this I tried background() function in draw() function too but it shows the changed color only at the start of application n again the color turns back to black.
Here is the some part of code:
////////////////////////////////////////////////////////////////
import msafluid.*;
//import fullscreen.*;
import processing.opengl.*;
import javax.media.opengl.*;

final float FLUID_WIDTH = 120;

float invWidth, invHeight;    // inverse of screen dimensions
float aspectRatio, aspectRatio2;

MSAFluidSolver2D fluidSolver;

ParticleSystem particleSystem;

PImage imgFluid;

boolean drawFluid = true;
//FullScreen fs;
void setup()
{
    size(1024,768,OPENGL);    // use OPENGL rendering for bilinear filtering on texture
    //    size(screen.width * 49/50, screen.height * 49/50, OPENGL);
    hint( ENABLE_OPENGL_4X_SMOOTH );    // Turn on 4X antialiasing
  //  fs = new FullScreen(this);
   // fs.enter();
    invWidth = 1.0f/width;
    invHeight = 1.0f/height;
    aspectRatio = width * invHeight;
    aspectRatio2 = aspectRatio * aspectRatio;

    // create fluid and set options
    fluidSolver = new MSAFluidSolver2D((int)(FLUID_WIDTH), (int)(FLUID_WIDTH * height/width));
    fluidSolver.enableRGB(true).setFadeSpeed(0.3).setDeltaT(0.03).setVisc(0.001);
     background(0,120,234);
    // create image to hold fluid picture
    imgFluid = createImage(fluidSolver.getWidth(), fluidSolver.getHeight(), RGB);

    // create particle system
    particleSystem = new ParticleSystem();

    // init TUIO
    initTUIO();
}


void draw()
{
    background(0,120,234);  //even doing this does not work
    updateTUIO();
    fluidSolver.update();

     for(int i=0; i<fluidSolver.getNumCells(); i++)
        {
            int d = 2;
            imgFluid.pixels[i] = color(fluidSolver.r[i] * d, fluidSolver.g[i] * d, fluidSolver.b[i] * d);
        } 
    background(0,120,234); 
   
    imgFluid.updatePixels();    
    image(imgFluid, 0, 0, width, height);
    particleSystem.updateAndDraw();
}

// add force and dye to fluid, and create particles
void addForce(float x, float y, float dx, float dy)
{
   // balance the x and y components of speed with the screen aspect ratio
    float speed = dx * dx  + dy * dy * aspectRatio2;
    if(speed > 0)
    {
        if(x<0)
           x = 0;
        else if(x>1)
            x = 1;
        if(y<0)
           y = 0;
        else if(y>1)
              y = 1;

        float colorMult = 5;
        float velocityMult = 50.0f;

        int index = fluidSolver.getIndexForNormalizedPosition(x, y);

        color drawColor;

        colorMode(HSB, 360, 1, 1);
        float hue = ( (x+y)*180 ) % 360;
        // float saturation = (x*90)%360;
         // float brightness = (y*90)%360;
        drawColor = color(hue,1,1);
        colorMode(RGB, 1); 

        fluidSolver.rOld[index]  += red(drawColor) * colorMult;
        fluidSolver.rOld[index]  += (drawColor>> 16 & 0xFF) *colorMult;
        fluidSolver.gOld[index]  += green(drawColor) * colorMult;
        fluidSolver.bOld[index]  += blue(drawColor) * colorMult;
        particleSystem.addParticles(x * width, y * height, 10);
        fluidSolver.uOld[index] += dx * velocityMult;
        fluidSolver.vOld[index] += dy * velocityMult;
    }
}
///////////////////////////////////////////////////////////////////////////////

If anyone have any idea then please suggest me how to do this.