True, the spectrum of sunlight is the best but problem may be the intensity and direction. I am having this issue right now....
I have what I call a hallway gallery. It's just my hallway where I display most of my own work. It has a near by window. While the light coming in is indirect, it's bright. Prints that look great there will be too dark anywhere else. Also, as the angle of sun changes, intensity changes as well.
As a stable source, I have one of those color balanced table light. It's a cheaper clone of Ott light type.
Whatever you do, if you intend to frame your photo, when you proof it, be sure to put it behind a pane of glass, and if known, view it at a location where you intend to display it. You'll be amazed how much the "feel" of your image change.