Quark doesn’t have a color picker or eyedropper tool to select colors. Even when you are creating new colors, there’s no way to select colors from imported images.
InDesign has an eyedropper tool in the Toolbox. With the eyedropper tool, you can select colors from a placed image. Click once to hold a color. Option click to select again. then you can go to the Swatches panel and choose New Color Swatch from the options menu.
The eyedropper tool does more than just color in InDesign – it can select formatting attributes. With the eyedropper tool selected, click on some formatted text and those attributes will be held. Click and drag over some unformatted text with the eyedropper tool and now that text will be formatted.
Yes, I suppose you should know the colors that you will be using in a given project ahead of time, but there are those times when you need to reference a color within your layout program without having to go back to Photoshop to find out it’s CMYK or RGB values. Apple has a utility called Digital Color Meter that you could use while in Quark to pick out values, but you’d still have to write them down and make new colors. Not as easy as InDesign.
This is a real ass-backwards workaround, but you can get color sample/eyedropper functionality in Quark: