We’re no longer sticking to the white notes, as this rule applies only to C major, and no other major scale. What matters here is the pattern created between each step of the scale. For these major ...

For instance, in a C major scale, C=1, D=2, E=3 and so on, until B=7. So, to build a C major triad, we need the notes numbered 1, 3 and 5 from the major scale; in other words, C, E, and G = C major.