The eyelashes usually have a seperate material setting, so to change the colour simply change the diffuse colour of the eyelashes mat region.

Brows that do not have a seperate region are a lot more difficult to change as they are on the texture so changing colour there would change the rest of the texture too.

Depending on your knowledge of either PS or PSP you can replace the eyebrows with skin on the texture and make new brows in a seperate layer which you can then colour to your likings.

Saving the file as PSD will give the option to change again later on as Poser6 does accept PSD files for textures.