There is no specific tool in Elements for choosing a range of colors like the Color Range selector in Photoshop. So, you need to create your own tool, which is surprisingly easy to do.
The Magic Wand tool has a setting called Tolerance, which defaults to a value of 32. You are not stuck with that value as it is a variable you can change to suit your needs. We can use this tool to select a range of values, and then modify the results. At the default setting of 32, if you click on a value of 200 in an image, the tool will select all the values from 168 to 232, or 32 values above and below your target. This is a hard selection in that value 168 will be fully selected and value 167 will be full unselected.
If we want to choose the highlight values in an image we can set the Tolerance of the tool to something like 80 and click on a very high value at or close to 255. This will select all values down to about 175 depending on the original target value, effectively selecting the highlights in the image. You could select shadow or mid-tone values in a similar fashion. If you click on a specific color, the selection will choose all the values within range for each of the three channels according to the values in each channel at the target point.
Now we can modify our selection in the usual manner with an adjustment layer whose mask will reflect our selection values. Your modifications in something like Levels will be rather harsh around the edges because of that hard selection of values I referred to, so we need to modify our results to make the process work for us without the funny appearance of the selected values against the unselected values. This is done by blurring the mask to effect a transition from selected to unselected areas. This blur in many cases will need to be rather large, so choose Filter | Gaussian Blur and drag the slider up until you see the modifications smooth out in the image.
Modifications like this work best if you overdo them to start with so they are easy to see, blur the mask, and reduce the adjustment layer opacity to control the amount of modification needed in the image. Painting on the mask with a soft brush or applying gradients from foreground to transparent will help you modify your selections to blend them into the original image.
When you modify the combined RGB values in an image with Levels or Curves your changes are not strictly brightness changes, even if that is your intention. They also modify the relationship between the color channels which result in shifts in color saturation and occasionally in the hue as well. This is easily brought under control with a simple change in the blend mode.
I burn corners with a curves layer, dragging the top or bottom end points depending on how I want to effect the image. Drag the bottom end point to the right and the image gets more contrasty as well as darkening the bottom end. Dragging the top point down darkens the image highlights and reduces overall image contrast. So, a combination approach usually works best, and mid-points can be modified as needed. If you darken a blue sky or other area with a strong color you will not only get a darker image, but a shift in saturation. Get severe with the changes and you can get a slight hue shift as well.
To remedy the situation change your blend mode to Luminosity. This restricts the effect of the changes to the brightness values in the image and eliminates the saturation and hue shifts. In most cases the effect is not radical, and if you like what happens, just leave the blend mode in Normal. I always check the effect to see which blend mode gives me the look I want.
The same control can be used anytime you modify something to lighten or darken with an adjustment layer. Quickly check the blend mode to see if a color change has occurred that may not be desirable.
There are other useful blend modes, but this one is especially useful and an easy step toward learning to use them to your advantage.
Do you find that having set black and white neutral points in an image, especially with Levels, modifying a mid-tone value with curves, such as a skin color in a portrait introduces a color cast? This is not surprising as the pivot points for the colors are at the far ends of the curves so there is nothing to prevent shifts. To prevent the shift you simply need to “lock down” certain values so that the pivot points are away from the ends and extreme values are protected.
To try this, open a curves dialog and place an additional point on the curve one block up and to the right of the bottom black point (assuming you are using the finer ten block detail in the curves dialog). Do the same one block down and to the left of the white point.
Those values will be at approximately 24 at the bottom and 230 at the top end. Additional points above and below these values can be added to lock in specific values if needed, and there is no rule as to where you lock down the values. Additional points help to minimize the shifts as the two initial points simply create new pivot points. Once your critical values are locked down, mid-range value changes will have a considerably lesser effect on the black and white neutrals.
Selection tools, especially Color Range and others with a tolerance setting will make pretty specific hard selections. I avoid feathering as it is not interactive or easily modified after the fact. I prefer to make a layer mask based on the selection and then blur the mask. Blurring the mask is interactive as you can see the effect the blur is imparting on the selection in the Gaussian Blur dialog, and other modifications can be made to the mask with Levels and painting.
Layer masks made with the Luminosity of a pixel layer or from a channel should also be blurred in most cases if you are making an adjustment or using a blend mode to alter the appearance of the image. An unblurred mask will reveal its existence somewhere in the image either as an edge or as a strange-looking veil over the image content. Blurring the mask eliminates this problem by blending the modifications into the image in a more subtle fashion.
The trick is in how much to blur, and that is a question without an answer. A hard edge along a building roof can be blurred as little as three-tenths of a pixel, essentially faking an anti-aliased edge on a mask where a sky has been changed, added or modified. In other cases, blending in content or a change of density can require a stronger blur of 2-5 pixels. In some cases rather more severe blurring is called for in an attempt to hide the effects of the mask. The answer is to boldly move the slider in the dialog and use the Preview check box to see what happens. You will find that there are few rules for the technique, but closing in on the proper setting is not really that difficult.
When I paint on a mask, even with a fairly soft brush, I generally check the results and add an additional blur at the end to see if the transitions become more believable and subtle. Not leaving evidence of your modifications is the goal.