I make a ton of lines, is there a way to set the default so that the lines I make are a certain thickness and color? I'd like to use inkscape to draw up diagrams while I'm talking, but it just doesn't work with me having to go in and change the width and color every single time!

I'd also like a way to make the default for arrow heads to be the same color as the line. Is there any way to accomplish these things???

By double clicking on the pen tool's icon you enter the tool's default preferencies panel, where you can set it's default style.
However you cannot define a default marker for line's ends.
Also colouring of them is only in the extensions -modify path/color markers to match stroke as far as I know.

You can group separate lines together, so that you can set a style for them at once.

Lazur URH wrote:By double clicking on the pen tool's icon you enter the tool's default preferencies panel, where you can set it's default style.

I'd brought this panel up accidentally before but had never realised how. Thanks for the tip.