Download internet pictures become low quality
I think it's to do with the resolution of the wall paper you downloaded - it's 1080 * 1920 image , which is not high enough to be scaled properly on your device without blemishes as noticed by you

Typical Hi resolution wallpapers which give a better result are in the region of 1240 * 2208. You could Google those for the solid colours you are looking for or use an app like Solid Colours ( I haven't tried that app but use a different app that gives jet black wallpaper )