Contrasting features can make a comparison more striking. The resemblance is not necessarily in the opposed qualities themselves; it may lie in the relationship, overall effect, or shared intelligible feature.
For example, pairing light and darkness in contrasting truth and falsehood emphasizes their opposition. Without a relevant common consideration, merely saying two opposite things resemble one another is not a reasoned simile.
Analysis must identify the contrasting traits and then the precise respect in which a comparison or correspondence is intended.