The error can be in your code in draw() or somewhere else. Hard to tell with this fragment of code. The " two stroke() references" isn't a problem because they are in different conditional branches: only one of the two will be executed in this part of code.

If your code is complex, test the logic with a simpler sketch, eg. drawing a rectangle with such stroke selection.
Also use different println() in both branches, to see where it goes.

i have sorted the stroke situation out. i removed one of them and i can not control the stroke colour if i imput it in the code stroke(255,0,0); turns it red.

What you've got there should be red when the button is depressed and black otherwise (or possibly the other way round depending on how you've wired your circuit).

so how would i go about putting this into processing. and thats exactly what i want to do is toggle between the colours. black being the start colour.

1. boolean isBlack; // A global variable to track if the square is black.
2. void setup(){
3.   size(200,200);
4.   isBlack = true; // The square starts off as black
5.   stroke(0);
6.   smooth();
7. }
8. void draw(){
9.   background(255); // Draw white background
10.    if( isBlack ){ // Is the square black?
11.      fill( color(0) ); // Yes. So fill it black.
12.    } else { // No.
13.      fill( color(255,0,0) ); // So fill it red.
14.    }
15.    rect(10,10,180,180); // Draw the square
16. } 
17. void keyPressed(){ // If a key is pressed
18.   if( key == ' ' ){ // and that key is the space bar
19.     isBlack = !isBlack; // toggle the variable
20.   }
21. }

You'll have to modify the logic a little bit, however, because you only want to toggle when the signal from your input was HIGH and then goes to LOW.

im not sure this is what im wanting to do its my line i want to change colour as i draw lines using my Potentiometers i then want to be able to change the line colour to red from when i press my button

That was an example of using a global variable to toggle a drawing setting. You can use the same logic for your line. You're just setting the stroke() color instead of the fill() color, and are drawing a line instead of a square.

ohh yea sorry i shuda looked properly. i changed the fill() to stroke() and it still didnt work?

What I posted is NOT based off of your sketch! It's an example. It's for you to learn how to program better, so that you can apply what you've learned from the example into your own code. It is NOT me trying to get your code to do whatever it is you want!

ARGH!

If you won't learn how to program, take this and go away:

1. int buttonPin3=7;
2. boolean isBlack = true;
3. boolean wasLow = true;
4. void setup() {
5.   size(200,200);
6.   arduino.pinMode(buttonPin3, Arduino.INPUT);
7. }
8. void draw() {
9.   // Draw background.
10.   background(128);
11.  
12.   // Determine color.
13.   if(!wasLow && (arduino.digitalRead(buttonPin3)==Arduino.LOW)) {
14.     isBlack = !isBlack;
15.     wasLow = true;   
16.   }
17.   if(arduino.digitalRead(buttonPin3)!=Arduino.LOW){
18.     wasLow = false;
19.   }
20.   // Set color
21.   if( isBlack ){
22.     stroke(0);
23.   } else {
24.     stroke(255,0,0);
25.   }
26.  
27.   // Draw lines.
28.   for(int i = 10; i<180;i+=10){
29.     line(i,10,10,180-i);
30.   }
31.   line(10,10,10,180);
32.   line(10,10,180,10);
33. }

ok thank you i will have a look and try and make sense of things. im really struggling to get to grips with programming.

ok i can see whats happening here.you have set co-ordinates of lines and when the buttons state is high its one colour but when its low it changes colour. but this is only getting set to them line co-ordinates.

You must draw the lines - with a call to line() - at some point in your program, right?
Before you draw the line, you set the color that the line will be with a call to stroke().
If you remember what color the lines should be by storing that color in a color variable,
you can then pass that color to stroke() before you draw each line.

1. // EXAMPLE CODE ONLY - WILL NOT RUN.
   
2. 
   
3. color myLineColor;
4. // ...
5. if( isBlack ){
6.   myLineColor = color(0);
7. } else {
8.   myLineColor = color(255,0,0);
9. }
10. // ... Whatever else you want to do or draw...
11. stroke( myLineColor ); // Use the remembered color.
12. line( x1, y1, x2, y2 );  // For this line.
13. // ... Whatever else you want to do or draw...
14. stroke( myLineColor ); // Use the remembered color.
15. line( x3, y3, x4, y4 );  // For this line too.