Why are the note heads olive green???

See Coloring of notes outside an instrument's range.

They are higher pitched than the "amateur" range. If you right click and look at Stave/part properties you will see near the bottom "Usable pitch range". These are specified at concert pitch. Obviously for a horn the ranges are rather subjective whereas for an oboe for example, you just can't physically get it to play lower than a Bb below middle C. You can modify what is shown in the boxes if you have a different opinion. Or you can leave them as they are. They will print as black in any case. The coloring is just to alert you during note entry that you may be going outside what can easily be played. If you go outside the "professional" range the note heads will show as red.