I guess it's in the eye of the beholder. I would argue that modern high quality tube amps don't sound colored or have a "signature" compared with solid state which sounds less natural overall.
Solid state tendencies:
-emphasize the leading edge or transient at the expense of decay
-overly etched sound and images that doesn't resemble anything heard in real life.
-a congested or clogged sound (when compared to the best tube amplifiers)
-an inability to resolve the space between the performers, making it sound more like instruments and voices in a black hole
-bass that extends down to DC but sounds overdamped and less natural
Now, all that said, there are some fabulous sounding solid state amps that minimize, to a great extent, all of these shortcomings, but they don't totally eliminate them when compared to the best tube amps.
In fairness, I will also admit that tubes have a couple of weaknesses of there own, but as Atmasphere has pointed out, they are less of a problem because they don't contradict the laws of human hearing.
Solid state tendencies:
-emphasize the leading edge or transient at the expense of decay
-overly etched sound and images that doesn't resemble anything heard in real life.
-a congested or clogged sound (when compared to the best tube amplifiers)
-an inability to resolve the space between the performers, making it sound more like instruments and voices in a black hole
-bass that extends down to DC but sounds overdamped and less natural
Now, all that said, there are some fabulous sounding solid state amps that minimize, to a great extent, all of these shortcomings, but they don't totally eliminate them when compared to the best tube amps.
In fairness, I will also admit that tubes have a couple of weaknesses of there own, but as Atmasphere has pointed out, they are less of a problem because they don't contradict the laws of human hearing.