Translation:Pessimists see dark colours, optimists see light colours.
This is wrong in English. It's a run-on sentence. There should be either a conjunction or a semicolon between the two parts. Syntactically, it's different from Ukrainian.
(The question is about "the dark colors" vs. "dark colors.") My guess is the sentence is about dark colors in general and not about some concrete dark colors. Therefore, the definitive article is not used.