how to solve the following problem

as your question inform that, you are using same css to both module, so if you change in css then it will affect to both the module.

so if you need to apply any specific change to particular module, just create a new css class and apply your desired css changes over there and use that class with your module, where you want that changes..

you can specified more than one class over any control like span, div, input etc.. so don't worries about it, just sap rate one new css class and applied Font Color over there and use that class to your First Module.