kontrasto

There is no -ast- suffix in Esperanto. The root is kontrast-, whole and unsplittable, and anybody trying to read it as kontra- plus something is about to build a word that does not exist.

Which is a pity, because the instinct is right about the history and right twice over. Latin contrastare is contra plus stare, to stand against, and Esperanto happens to have taken both of those words separately as well: kontraŭ at rank 144 is contra, and stari is stare. So the two halves of the Latin compound are both sitting in your vocabulary already, in their own right, and the word built from them still is not kontrasto. Kontraŭstari is a real and useful Esperanto verb meaning to resist, to stand up against something. It is not the same idea at all.

This is the same shape as neglekti and reflekti in this band: an honest Latin seam that does not transfer. Three of them arriving together is not a coincidence. Late Latin and its descendants supplied Esperanto with a large stock of learned verbs and nouns that were already compounds when they were borrowed, and Zamenhof took them as units because that is how the international vocabulary had them.

Fences. A diferenco is any difference, however small or dull. A kontrasto is a difference sharp enough to be worth pointing at, and it is the word for the visual property of an image or a screen. The adjective is kontrasta, the verb kontrasti (to contrast, intransitive: la koloroj kontrastas), and en kontrasto al in the third example is the ordinary way to say "in contrast to", though male (on the contrary) is shorter and often better.