"I like to read near the window because it is bright there."
Translation:Я люблю читать около окна, потому что там светло.
"Ярко" can be used to describe something that emits strong light, like a lamp, fire, the Sun. It can also be used to describe bright, vivid, strong colours or sound (antonym to pale).
In this case we're just talking about a place that has enough light for reading while the other places in the room are too dark for that (or just darker). So it should be "светло".