ofVec3f the closest value in the array

right now there's definitely one problem here:

char id;

it's not really set to anything. in c++ variables don't have an initial value, which means that they are set to what ever is in memory at their location. This can lead to spurious values and really hard to debug situations. I can see your code failing when id isn't set to anything new.

usually what I do it set a variable to a value that's easy to check against, ie:

int id = -1;

then, I search, and if I find what I want, I set id to that new value. that way I can always check if the return value is -1 and check my logic, etc.

in your case, you might want something like a linear search for the minimum value:

    int closestId = -1;
    float minDist = 100000; // start with a large value;
    for (int i = 0; i < NUM_CHART_MAP; i++) {
      ofVec3f rgb = colors[i].rgb;
      float dist = (rgb - colors[i].rgb).length();
      if(dist < minDist) {
           minDist = dist;
           closestId=colors[i].id;
      }
    }
    return closestId;

hope this helps!