TL;DR
The new coal ore graphics are harder to see at a glance due to poor contrast. Make them darker like before 0.17.46What ?
It is an important part of playing Factorio to be able to assess the state of your factory at a glance from a higher zoom level. Consider the following image:
The flow of iron ore, copper ore, and stone ore is easy to see visually even if you were quickly scanning across the map. However, coal ore blends into the background due to low contrast. Also, it just looks dull. The darker and more vibrant coal ore that used to stand out from the background and other ores is now replaced with a dull version that is hard to distinguish visually and easy to miss.