1) The profile of the rim/side transition must be exactly a certain contour (slightly parabolic, with fully feathered-in edges); 2) the surface of the rim itself cannot be---believe it or not---truly flat, but ALSO faintly parabolic in cross section (wtf, right? lol ); and 3) the color must match under all sources and temperatures of artificial light---flourescent, incandescent, halogen, etc.---as well as when viewed outdoors in sunlight. i.e. "full spectrum" light.
It's a complete, absolute, royal pain-in-the-ass pig bitch to dial in those three things, but it CAN be done, so hang in there.
For the correct contours, have an unmodified bowl of the same size and shape handy to check against while you work; and for the color problem, use a full-bandwidth light source that "behaves" (so to speak) like sunlight. If you don't, what looks like a perfect match under one type of light will look laughably off when viewed under another.