i found that though all these tools like inkscape are quite good at what they do, they aren't that useful in the prototyping stage, nothing beats the classic paper-pen combo. or maybe it's about the approach of getting the dimentions and size right and then starting the design process, probably that. or learn the tools so well that the overhead of using that for making prototype designs over paper is minimal to none.
as a developer, it's quite important to not design things for yourself but for the users. the term users covers a vast demographic from ones who are used to computers to the ones who don't have any idea about computers and are trying to understand what all the options in the user interface mean.
defining a color palette before hand is quite important as then you can't mix match colors in a way that they make the interface look ugly or chaotic, unless the palette was selected just for that.