I have written more than a few times that I typically use the Z key to evaluate image detail for sharpness, noise, etc. Especially when comparing two similar images. I do this because it's faster for me than using the loupe tool. So does that leave the loupe tool sort of orphaned and unused, for me the answer is no. I use the Loupe tool mainly for color evaluation and manipulation.
This Aperture 3 super simple stuff post is a two fold message - the first being about the loupe tool, the second being about color numbers in general. Way back in the olden days there were two different kinds of color numbers that people looked at, RGB and CMYK. The only people that lived and died by these numbers were color separation geeks. They had not much choice. For some ridiculous reason the photographic community sort of latched on to these numbers and tried to make them meaningful when evaluating and processing images. The reality is they are not intuitive to deal with when trying to get anything done photographically. Thats because all components of that number, RGB, change for every single thing you change from a photographic perspective. Want to change saturation, all three numbers change, luminance, all three numbers change, actual color value - same - all three numbers change.
Here is where HSL color numbers come in very useful. Each one is completely independent for the other numbers. No matter how saturated or bright a color is the hue number is the same if it is the same color, etc. This is great because you also have a color control block that manipulates these values directly for any color, combination of colors, or range of colors. So here is how to use the loupe as a handy tool to inspect that HSL number.
Step one, make sure that the loupe tool is setup to display color information.
Step two, using the gear menu in the adjustment inspector set the color information display to HSL and use a medium or broad sample point
Step there, use the back quote key to toggle the loupe on or off like this and enjoy easy to reference, easy to comprehend color information that correlates directly to the color adjustment block sliders.
Ps. Latest in the Aperture 3 Super Simple Stuff™ series.