Just to show how #6 looks in different colors: http://imgur.com/a/jvVpL
The simple D would stand out on exchanges etc. as shown in the example below. If we go with a 'standard' coin design it will be hard to differentiate DRK from other coins based on looks.
Nice ! Ok maybe it's crazy but @LimLims why not use all those color variations like @NameUser proposed or at least some of them. I can imagine t-shirts with those different logos. The D is recognizable enough.