There's a bit of lag between green and cyan. The green values don't decrease immediately when the if statements called. I put in a print statement to check the values, but they seem to stick at 255 for while before decreasing.

If anyone can offer any help pointers, or advice for a better method to do this, with the exception of using libraries, it would be much appreciated; thanks

P.S. Can this be done with for loops?

   //Colour tween animation
   //Set starting colour values 
   int r = 255;
   int g = 0;
   int b = 0;
   
   
   //Control rate of colour change
   int speed = 4;
   
   void setup() {
     size(400, 400);
   }
   
   void draw() {
   //Assign int values to background
     background(r, g, b);
   
   //Start, increase green RGB value
   //Red to yellow
     g = g + 1;
   
   //Yellow to green
     if(g >= 255) {
       g = 255;
       
       r = r - speed;
     }
     
     //Green to cyan
     if( r <= 0) {
       r = 0;
       
       b = b + speed; //lag between change
       println("green: " + g);
     }
     
     //cyan to blue
     if(b >= 255){
       b = 255;
       
       g = g - speed;
      }
      
      //Blue to purple
      if(g <= 0){
        g = 0;
        r = r + speed;
      }
      
      //Purple to read
      if(r >= 255){
        r = 255;
        
        b = b - speed;
      }
   }