The white outer ring with dimmer-looking center is usually caused by Mach banding. It’s not a flaw in the beam; it’s a quirk of the way humans see.
When brightness changes in a linear pattern, it makes a sharp “corner” at the beginning and end of the line. This corner then appears brighter or dimmer than everything around it, even though it’s not.
Here’s a diagram showing the actual vs perceptual brightness of Mach bands:
… and when the pattern shows up in a flashlight beam, it usually looks like this:
The hotspot should appear as if it has a white ring around the edge, with a darker inside. The center may even appear concave like a bowl. But it’s not. It’s perfectly flat.
If you have an image viewer with an eyedrop tool, feel free to check the image. The entire hotspot, including the “brighter” edge, is a single color – #e6e6e6. The white ring and concave center exist only in your brain.
That doesn’t mean the beam is perfect though, even if it could technically be described that way due to a completely flat hotspot. Focusing like this is a great engineering feat, but doesn’t look very good. Instead, an ideal beam should be tuned to match human perception.
To avoid the Mach band effect, the changes in brightness should generally follow sine curves instead of lines. This is one of those weird cases where a straight line looks curved, and a curved line looks straight.
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As for the color of different parts of the beam, that’s a completely different effect with a different mechanism. Mach banding doesn’t affect the color; only the brightness.