Color change a background in processing w/ out affecting particles - background

Question

I'm trying to fade a background in and out into different colors. I decided to try drawing a rect rather than using background, but I'm getting trails on my particles, because the background isn't getting drawn. Suggestions on how to fix this?
// main tab
ArrayList<ParticleSystem> systems;
PVector windRight = new PVector(0.1,0);
PVector sortaSpeed = new PVector(-0.1,0);
PVector gravity = new PVector(0,0.05);
boolean wR = false;
boolean sP = false;
boolean cS = false;
int limit = 8;
int alpha = 10;
color[] colorArray = {color(0,0,0,alpha),color(16, 37, 43,alpha),color(51, 10, 10,alpha),color(126, 255, 0,alpha)};
int currentColor;
int nextColor;
boolean change = false;
void setup() {
size(640,480);
systems = new ArrayList<ParticleSystem>();
smooth();
noStroke();
currentColor = 0;
for(int i = 0; i < limit; i++){
systems.add(new ParticleSystem(random(100,200),10,new PVector(random(100,500),-5))); //random(480)
}
background(0);
}
void draw() {
//rect(0, 0, 2000, 2000);
fill(colorArray[currentColor]);
rect(0, 0, width*2, height*2);
//background(colorArray[currentColor]);
if(change){
currentColor = nextColor;
change = false;
}
if(!systems.isEmpty()){
for (int i =0; i < systems.size(); i++){
ParticleSystem ps = systems.get(i);
ps.applyForce(gravity);
ps.run();
if(wR){
ps.applyForce(windRight);
}
if(sP){
ps.applyForce(sortaSpeed);
}
}
}
}
void keyPressed() {
if(key == 'w'){
wR = true;
print("w");
}
if(key == 'a'){
print('a');
sP = true;
}
}
void keyReleased(){
if(key == 'w'){
wR = false;
} else if(key == 'a'){
sP = false;
} else if(key == 's'){
if(key == 's'){
change = true;
println("currentColor: "+currentColor);
int newColor = currentColor;
while (newColor == currentColor)
newColor=(int) random(colorArray.length);
nextColor = newColor;
}
} // end of cS
}

Answer 1

In the future, please try to post an MCVE. Right now we can't run your code because it contains compiler errors. We don't need to see your entire sketch anyway though, just a small example that gets the point across.
But your problem is caused because you're trying to use alpha values in your background. That won't work with the background() function because that function ignores alpha values, and it won't work with the rect() function because then you won't be clearing out old frames- you'll see them through the transparent rectangle.
Instead, you could use the lerpColor() function to calculate the transition from one color to another over time.
Here's an example that transitions between random colors:
color fromColor = getRandomColor();
color toColor = getRandomColor();
float currentLerpValue = 0;
float lerpStep = .01;
void draw() {
color currentColor = lerpColor(fromColor, toColor, currentLerpValue);
currentLerpValue += lerpStep;
if (currentLerpValue >= 1) {
fromColor = currentColor;
toColor= getRandomColor();
currentLerpValue = 0;
}
background(currentColor);
}
color getRandomColor() {
return color(random(255), random(255), random(255));
}