The problem is indeed as calsign indicated. Additionally I would like to mention that this method is a waste of resources, because you check for each position/counter against the whole array (until a match is found). So the bigger your fibo array is the more of a waste this method is. Instead, since your fibo values are in order, you can check against a single value, the first upcoming fibo value. Also, your code skips 0 as a displayed number. Also I removed hard coded values so it works with all dimensions (width, height, dim).

Adapted Code

1. int[] fibo = new int[30];
2. int dim = 100;
3. int fiboIndex;
4.  
5. void setup() {
6.   size(500, 500);
7.  
8.   int fibo1=0, fibo2=1;
9.   for (int i=0; i<fibo.length; i++) {   
10.     fibo[i] = fibo2 + fibo1;
11.     fibo1 = fibo2;
12.     fibo2 = fibo[i];
13.   }
14.  
15.   float w = (float)width/dim;
16.   float h = (float)height/dim;
17.   for (int y=0, counter=0; y<dim; y++) {
18.     for (int x=0; x<dim; x++) {
19.       if (isFibo(counter)) { fill(255, 0, 0); } else { fill(255); }
    
20.       rect(x*w, y*h, w, h);
21.       counter++;
22.     }
23.   }
24. }
25.  
26. boolean isFibo(int c) {
27.   if (fiboIndex>=fibo.length) return false;
28.   int fiboNumber = fibo[fiboIndex];
29.   if (fiboNumber == c) {
30.     fiboIndex++;
31.     return true;
32.   }
33.   return false;
34. }