How to evaluate student team members who did minimal work on a team assignment?
Evaluating your fellow students should be a supervised exercise of constructive criticism, e.g., after a presentation given by them. I hope this is what the lecturer will do.

Anyway, if you are assigned this you need to do it. I would give the bare facts for the students that didn't really involve themselves. I.e., I would state exactly what each team member contributed to the project. And if they only suggested a color for a button then I would state that. I would then proceed with providing an honest evaluation of the student that did all the work with you and for the others just say that they could have involved themselves more in the project.

I know that this feels a bit like being a tattletale, but it is actually them that behaved unethically by not working with you on the assignment. The only thing you should have done differently is having a frank discussion regarding their involvement (with them and with the lecturer) at an early stage. Resolving issues like this during team work is actually something you need to learn during your studies. And it appears like you haven't. Thus, you should add this as self-criticism to the evaluation and ask the lecturer to teach you methods of handling such issues.