  Another answer would be that scarlet tropes form a primitive natural class (this view has been forcefully defended by Ehring 2011: 175-241). But whether or not what makes scarlet tropes scarlet tropes is that they resemble each other, scarlet tropes do resemble each other.  And the fact that they do raises an important problem. This is the problem of the resemblance regress. Suppose that a, b and c are scarlet apples. If so, each one has its own scarlet trope: call them sa, sb, and sc. Since sa, sb, and sc are scarlet tropes, every two of them resemble each other. But then there are three resemblance tropes as well: the resemblance between sa and sb, the resemblance between sa and sc, and the resemblance between sb and sc. But these resemblance tropes, since they are resemblance tropes, resemble each other. So there are ‘second-order’ resemblance tropes: the resemblance between the resemblance between sa and sb and the resemblance between sa and sc, the resemblance between the resemblance between sa and sb and the resemblance between sb and sc, and the resemblance between the resemblance between sa and sc and the resemblance between sb and sc. But these ‘second-order’ resemblance tropes resemble each other. So there are ‘third-order’ resemblance tropes, and so on ad infinitum.