ofVec3f the closest value in the array

you can also do:

ofVec3f col;
auto nearest = std::min_element(colors.begin(), colors.end(), [&](Colors&c){
    return c.squareDistance(col);
});

which returns an iterator to nearest element in the original vector so later you can do:

nearest->x = ...;

or

ofVec3f v = *nearest;