I am also wondering if there is a tool that allows you to define a portion of capture based on the existing color scheme and then assign a totally different color to it. For example, if I were to take a photo of a red apple against a white background with nothing else whatsoever in the photo, and lets say i wanted to pick the red of the apple and assign a totally different color to it, say make the apple green or yellow instead. Is there such a tool that allow such an operation?
There are various tools that will assist
you in making the sort of edit that you describe. In many cases, like a red apple against a white background, the tools can make the process almost automatic. In other cases, the process can be very labor intensive to get a good result.
As I mentioned above, if you use the blending options for layers in Photoshop, you can apply an edit only to regions of a specified color range, such as white or red. There are also tools in Photoshop that allow you to select areas based on color or tonality and then apply edits only to the selected areas. There are whole books written about how to do that. There are also software add-ons sold that assist in the process. Basically, Photoshop is the tool of choice, and depending on the particular image there are often multiple ways to approach the task.
In Lightroom (or Adobe Camera Raw) there are several tools that may work for some images. The Adjustment Brush has an Auto Mask feature that will assist you in applying adjustments just to a particular object--it would probably do a decent job of selecting your red apple. Also the HSL tools allow you to make adjustments that apply just to particular color ranges. But overall, Lightroom and ACR are not the tools of choice for the sort of edits you're talking about.