The three most probable causes I can think of for the brownish tint would be:

1. That you're using Skylight filters, instead of something color-neutral, like a UV filter.

2. That you are shooting JPEG and you haven't set your white balance correctly.

3. Thorium or some other issue has caused your lenses to brown over time.

White Balance

The camera is actually doing some digital processing on the sensor data to create your image files. And if you are shooting JPEG files, it's not only compressing the image data, it's also using settings in the camera, such as your white balance setting to create the file.

The white balance can be set to offset common color-cast issues, such as shooting under incandescent/tungsten lighting giving an orange cast. This white balance processing would add blue to the image to try and shift the colors back to something more neutral.

You could consider shooting RAW or RAW+JPEG, to give you a chance to "reset" the white balance in post-processing.

Thorium (aka, are your lenses radioactive? :-)

Many lenses made from the '40s to the '70s use thorium glass elements. The Pen F 40/1.4 is one of them. Thorium is mildly radioactive and browns or yellows the glass as it ages. This tinting can be reversed by dosing the lens again with UV radiation to reverse the decay. Simple LED desk lamps have more than enough UV to do this, as does putting the lens in sunlight. But obviously, you want to take care you don't use direct sunlight in such a way that your lens becomes a magnifying glass burning a hole in something, or that the lens heats up enough to affect any plastics/grease/adhesives in the lens.

See: http://camerapedia.wikia.com/wiki/Radioactive_lenses