Dave,

To create your own custom mask:

1. Apply the Adjustment you want to an image, and brush something in. I would advise that you quite literally “write” something with the brush, like a big X, so you know which one you’re looking for (this will make sense in a moment).
2. Save that as a preset.
3. Quit Aperture
4. Go to ~/Library/Application Support/Aperture/Adjustment Preset Brushing
5. Put it in List View (Command-2), and select all, then Command-Opton-Right Arrow, which will open all folders and all enclosing folders.
6. You will see a TIFF file for every mask you currently have saved as a preset. Sort by size to make the non-empty folders go to the top. Select one of the TIFF files at a time, tap the Space Bar to QuickLook open it. Keep doing this until you find the mask you just created (this is where you’re looking for the X that you drew)
7. Once you find it, open that TIFF in Photoshop
8. Paint a gradient, or whatever you like, on that file.
9. Save and close.
10. Open Aperture, apply that preset, and voilà, there’s your custom preset.

-Joseph @ApertureExpert

Dave,

Apologies for the delay in responding. I’m traveling at the moment.

The gradient presets are gradient masks applied to the Tint adjustment. If you scroll down the Adjustments panel after applying the preset, you’ll see that the Tint adjustment has been added. You can drag the tint slider there to adjust the color added to your image.

-Joseph @ApertureExpert

Dave,

Oh wait… you said a neutral gray. Unfortunately the Tint will always tint it… not just darken it.

What you probably want to do is apply the Polarize filter, and brush that in. Unfortunately I don’t have gradients applied to the Polarize (or any other) adjustment at this point. It turns out that Aperture suffers a performance hit on many systems when you get too many adjustments with masks applied (which is what these are), so I’ve been reluctant to release any more. Which is a shame—I’d love to make hundreds of these available, but I think it would bring everybody’s Aperture to its knees if I did.

You can create your own—if you want to know how, I can do a write-up on it. But it does require Photoshop.

-Joseph @ApertureExpert