Coding amateur here in need of some help! 

I want the stroke of my shape to cycle between black(0) and white(255) continuously. I am stuck with the logic for this.

So far I have used a boolean variable of 'ascending' for when the stroke  var colour is ascending, and using an if statement to cycle the colour up by increments of one if ascending is true. I have also managed to make ascending to become false when the stroke colour reaches 255 (white).

I now want to make the colour decrease when it hits 255 but can't seem to get this to work. Please help!

2. b
3. boolean ascending = true;
4. int colour = 0;
5. 
   
6. void setup()
7. {
8.  size(800, 800);
9.  strokeWeight(5);
10. }
11. void draw()
12. 
    
13. {
14. 
    
15. if (ascending); {
16.   stroke(colour++);
17.   if (colour == 255) 
18.   {
19.     ascending =false; }
20. 
    
21. rect(200,200,200,200);
22.     
23. 
    
24. 
    
25. }
26.   println(colour);
27.   println(ascending);
28.   
29. 
    
30. }
31. 
    
32.