If you're using the felt pen with art pen mode, I imagine that you currently have it set so that your colour picker main area is set to grey ( lower right corner area ) and that the vertical luminance slider to the right is set to black ( lowest setting ). At this point, your pen will be very black as you've noticed.
If you want to make the pen a little lighter, you could move the luminance slider up a little in the right hand bar until the grey/black is about right for what you want.
The marker is basically a felt pen tool, so by default the black has a bit of translucency to it. When the art pen option is enabled it more closely follows the value you've selected, but if this is too dark, setting the luminance up a bit should enable you to get a bold look without it being quite so black. However it won't have the same translucent qualities of the standard setting.
Hope that's of some help!