All 16 colored terracotta textures were not updated in 1.14 alongside regular terracotta. Most likely they were forgotten about and weren't updated. I've attached a diagram better showing this.

What you'd expect:
The colored terracotta textures use the 1.14 texture as a base.

What actually happens:
They use the programmer art texture as a base.

The issue is not the colors (it's not what the bug report is about), just the texture they use as a base.