Color has a tendency to fool the eye into thinking that the contrast levels are okay. The best method I’ve found to insure that there is sufficient contrast is to take an existing image and convert to grayscale.
By looking at the tonal values alone, without the distraction of color, I can see if the range of dark to light shades provide sufficient contrast. A poor tonal range will yield grayscale values that are very close to one another. A good tonal range will have areas of high and low contrast to balance the composition’s midtone grayscale values.
Below is the prairie illustration with all of the color information removed. There appears to be sufficient contrast within the piece. If I did find low areas of tonal contrast, I could make adjustments to fix it. To quickly desaturate the image in Photoshop, bring up the Hue / Saturation palette (Command-U) and adjust the saturation slider to the far left. Another method to quickly desaturate a given image is to first flatten the layers (Shift-Command-Option-E) and select Image > Adjustments > Desaturate. Command-Shift-Option-E will collapse all the existing layers into one layer, while preserving the original layer structure (if future edits are required). I prefer this method of flattening instead of using Command-E.
Below is the final version of the prairie illustration for comparison purposes. To give the image a depth-of-field effect, select Filter > Blur Gallery > Iris Blur from the Photoshop pull down menu.