Color Separation
A single yellow bloom will really pop in a bush of red ones, even though they are all roses.
One of the challenges of writing for homogeneous instruments is creating enough aural space around a solo or melodic line. A horn solo buried in a harmonic pad played by the other horns isn't going to stand out as well as that melody written for piccolo trumpet over a low brass pad.
This is a particularly thorny problem with the middle register (alto, tenor, baritone) instruments. If you are writing an orchestration with a tenor or baritone register solo, there are several options available (trombone, euphonium, horn, low trumpet, high tuba).
Below are the first 4+ measures of Percy Grainger's original setting for piano of Irish Tune from County Derry:
The first consideration must be which of the available instruments best serves the music.
When Grainger set this for military band in 1918, to make certain the melody had enough carry his solution was to score it for multiple baritone range instruments (alto clarinet, baritone sax, horn IV, 2 trombones, and baritone). With that crew on the job, there was very little chance of burying the tune.
I chose to make this a euphonium solo in my brass ensemble orchestration. Nothing earth shattering about that decision, but I could have made others (doubling up on the tune like Grainger e.g. horn or horns, trombone or trombones, tuba or tubas or any combination of those instruments). I decided solo euphonium.
Side bar: euphonium is tricky and misunderstood. I love the sound and a good player can make it sing through a group. But like the flugelhorn it loses some of its characteristic sound if it has to fight through texture to be heard. This is especially true for notes written in the bass clef staff.
Once that decision was made, choosing the accompanying voices had to be done with care. The possibilities are nearly endless, but a casual choice of convenience might have been a poor one. The piano score suggests tuba and trombones with maybe a horn or trumpet on top. I thought that would be too heavy, so I left the trombones out of the mix and went with a conical choir voicing the horns in the low register below the euphonium and flugelhorns on the notes written above the tune. Also, I wanted to save the trombone and trumpet choir sound for the opening of the second verse of the tune - using orchestration to reveal form.
Where the voicing above the tune thickens, I used B flat trumpets with a written direction in the part to blend with the horns. On the face of it, that's a silly comment because the horns are written pretty low there and 'blend with flugelhorn' might have been a more obvious direction. But I wanted the trumpet players to think low and round.
Also note - I raised the key a step from Grainger's piano setting. Grainger also raised it a step when he scored the piece for military band. I suspect like Grainger, my decision was also made due to the tessitura of both the melody and accompaniment. It may not seem like a big deal, but one step accomplished several objectives. A subject for another day.
There are many solutions to creating the necessary aural space - dynamics, articulation, tessitura, mutes, making use of available varieties of colors inherently available in the ensemble (subtle as they may be).
The point is - you can't simply transfer material from the original into your brass ensemble score without being sensitive to balance and color. Sometimes the obvious choice IS the best one, but not always. Hear the sounds in your head and DECIDE.