In general, evidence suggests that we are attracted to contrast rather than brightness (luminance). For example, the onset of a bright light on a dark background is often used as an exogenous cue for visual attention, but a dark cue on a bright background works just as well. So, signs generally aim to be high contrast against their background. Contrast shows where things change, and hence it is picked up by the visual system for determining edges and objects of importance.
A more specific answer to your question is provided by Vincent et al., 2008. This paper, "Do we look at lights?..." includes discussion of the debate about whether visual saliency attracts attention and eye movements. The short answer is that, while saliency does correlate with where we look, on average and in specific circumstances, it depends a lot on our current task demands and the context. You won't look up at the sun if you are looking for your friend in a crowd.