Butter yellow walls don't usually suggest 'modern'. It depends on what the room looks like overall.

well it will be remodeled and right now have only the butter yellow color decided....I think it's an warm color and it tells contemporary/modern if you model it that way...now my questions was about the couch/sectional color...I know beige works perfect but not when you have children...so I am torn between black or brown....

Since you don't want a light colored sofa, (my first thought was white or cream) I'd suggest a dark gray or even navy for something different. I don't like black with yellow unless it's a bumblebee :), and brown with yellow is less of a modern color scheme. If you use dark gray or navy, adding white to the room in drapes or accessories will keep it fresh and crisp.

Yes drapes will be white linen....and yes it will have white accessories...a white stone fire mantel as well...I do like the idea of dark gray :):)

Butter yellow walls, charcoal or dark grey sofa, white linen drapes, the FP... sounds like a very nice foundation. Good luck as you put the room together, send pics:)