There are generally two approaches to the philosophy of colour in recordings - as I see it.
One: You're on a budget. Therefore, you want to get gear that will be as transparent as possible. The Grace 101 mic pre was reviewed in Recording Magazine, and the reviewer was amazed at how transparent it was. There is a fair amount of gear out there that is relatively affordable and reasonably transparent. Here, you rely on the colours of the instruments alone to add character to your recording.
Two: You can afford a wide range of gear, including things like Sennhaiser U87 mics, a Neve console, LA2A compressors, etc. In fact, you have a collection of all sorts of expensive stuff. Why have all this stuff? Because much of the expensive gear is highly saught after because of HOW they colour the sound! This is why some instruments/singers will sound better or worse with different combinations of mics, mic pres, etc.
Here is an illustration of the difference....
One: Cool.... that sounds exactly like me.
Two: Holy cow! That recording really makes me sound good!!
As an aside...
There is a ruler-flat mic, according to Harvey Gerst (aka Mr. Mic Guru) on "the big thread." It is made by B&K and it was made for test purposes. They're rarely used in recording, though, because they are percieved as "boring" mics.
For more info, check out "the big thread"
here
It is quite elaborate and involved, but it is so valuable that you should have to pay for it. Really.... check it out!
Chris