Collision between registered trademark and app branding

The short answer is "It depends". In many countries a registration of a mark can effectively block the entire class; the United States takes a much more granular approach when evaluating likelihood of confusion. A full analysis needs to be determined between the anticipating mark and the proposed mark as mentioned in the earlier post. From a registrability view point, the USPTO evaluates likelihood of confusion using its factors, while the District Courts, depending upon the circuit, use other factors in making the analysis.
As suggested in the first response, the situation needs to be evaluated to make a determination of the relative risks associated.
For a start-up company, the last thing that you want to do is walk into an infringement situation -- it will cost your company a bunch of time, and money, diverting important resources to something that is usually unproductive. I suggest that you consult with an attorney to discuss the specific details. Please feel free to contact me directly with details of the circumstances and i would be happy to look at the situation for you.
Gordon