I am sure that this subject may be subject to flames at one point or another, but maybe there is something to learn here! My take is where do you want to sound dark: to yourself, to the audience, to a microphone? Is playing dark even sensible (dark sound in marching band can mean poor projection)? A lead player with a "dark" sound will have trouble putting "edge" on the brass ensemble sound. So first we define where you play and if dark is suitable there. If yes, we need to define what dark means. The overtone series on a trumpet is mathematically fixed. All we can do is change the balance of them. More overtones means brighter and there are various ways to change them. Embouchure, mouthpiece cup depth, bracing on the horn all can affect the overtone structure. A hardened bell will vibrate and give the player bright feedback. A heavy, annealed bell will focus more energy in the audiences direction giving the player much different feedback. We also have to consider how we practice. If our room is small and has hard walls, we are basically piping the trumpet sound right into our ears. That sound is bright regardless of how one plays. If we try and adjust our tone for the practice room, our live sound in decent acoustic spaces can end up VERY stuffy. So, what to do? Like any other intelligent decision, we need to figure out where we are NOW. Go to the type of room you normally play concerts in and record yourself from various positions. It is very possible that your sound is already just fine. If not, then you need to make sure that the basic mechanics of playing are OK (breathing, long tones slurs, articulation), then perhaps an equipment review is in order (when all other factors are deemed satisfactory!). Getting a bigger sound is easiest when one practices in bigger good sounding rooms. Believe me, if a symphony player practiced for 4 weeks ONLY in a small bedroom or office, they would have a much rougher time on stage. Their rehearsal schedule means that they are playing a great portion of the time in big rooms where projection and sound concepts can develop! I think that many times players say that their concept is dark, because they heard somewhere that that is cool. None of the major symphony players that I have ever had the pleasure of knowing or hearing, has had a "dark" sound in my opinion. Adjectives like "brilliant", "massive", "delicate", "full" are more like what reaches my ears. Dark sounds I hear when certain jazz players blow right into the microphone on some smokey chorus. 10 minutes later, they can wail without the mic in the brass section. That variety of color is my goal. My sound concept is "robin" and that is dark, light and everything else.