sea, mountains and sky
A landscape is stitched the way you look into the distance: in broad areas of colour. Here are the ones the community has already charted.
A sky is built in bands: four or five blues, from palest to deepest, are enough. More shades make a sky softer on screen, but on fabric they become hard to tell apart, and hard to follow on the chart.
A landscape needs width: 120 stitches or more for a horizon to breathe. On 14-count fabric that is about 22 cm, a fine piece to frame.
DMC references that come up often on this subject, each with its own page.