Should I mail my advisor concerning his attitude towards me, and possible re-use of my ideas by a colleague?
I have contacted already a third person about this (edit: an ombuds-person, who is also a professor at my faculty)

This ombudsperson is probably the person best suited to give advice about whether misconduct occurred and what your options may be. Given that you are already talking to an ombudsperson, I would not want to add my own speculation into the mix.

[I] am thinking about sending to my supervisor regarding my disappointments and questions about this issue later on, once I finished my exams and obtained my master.

Your instinct to wait until after finalizing your degree is probably a good one. As for the advisability of writing such a letter at all: well, I am afraid I do not see much point. Certainly it seems unlikely that your advisor will say &quot;Egads! I have seen the error of my ways and am overcome with remorse!&quot; Your desire to protect future students is admirable, but unless the ombudsperson recommends further action, I suspect your options will be somewhat limited.