THIS ISSUE IS FIXED IN 19w42a SNAPSHOT

The Bug

If you are creating a new world and you switch the world type to superflat, you can customise the world as usual. However, in the 1.15 snapshot, you cannot see the layer icons or preset icons. This happens if you haven't played in any worlds after you launch the game. If you have played in a world after you launched the game, the icons will appear, but they will be colorless. 

How to reproduce it

1: Open Minecraft, if not already open.

2: Click singleplayer, then click create new world.

3: Change the world type to superflat.

4: Click Customise. The layer icons won't be there.

5: Click Presets. The preset icons won't be there.

If you play in a different world before you reproduce this, the icons will appear, but they will be colorless.

I use a 2017 Macbook Air. The graphics card I use is Intel HD Graphics 6000 1536 MB. If you need to know, my startup disk is Macintosh HD and my memory is 8 GB 1600 MHz DDR3.