What is a Eb neck? and what is a D neck? And how can I make my bass sound more brighter?

'Eb' and 'D' as applied to necks refers to the point at which the neck stock curves toward the back of the bass; as you slide up the neck, when your thumb hits the curve, your first finger will be at D or Eb on the G string. (Approximately. This can be upset by placementof the bridge.) Playing in the upper positions becomes unwieldy sooner on a D neck.
Brightness is affected by many things. Without complete knowledge of your set-up, it's impossible to say.