ɪn ðɪs kˈeɪs, ðə tˈuː ˈædᵻd vˈɛɹɪəbəlz ɑːɹ vˈɪʒuːəl ˈɪnpʊts sˈʌtʃ ɐz kˈʌlɚɹ ænd bɹˈaɪtnəs bɪkˈʌz ðeɪ ɑːɹ nˈætʃɚɹəli tˈuː vˈɛɹɪəbəlz ˈiːzili pɹˈɑːsɛst ænd dɪstˈɪŋɡwɪʃt baɪ ðə hjˈuːmən ˈaɪ.
In this case, the two added variables are visual inputs such as color and brightness because they are naturally two variables easily processed and distinguished by the human eye.