[Probuilder] Brush Color palettes in vertex color tool appear white.

Steps To Reproduce

1. Create a project
2. Window -> Package Manager -> Install Probuilder
3. Tools -> Probuilder Window
4. Probuilder -> Vertex colors -> Open Vertex Editor

Expected Result: Brush Color palettes should be of the respective color swatches

Observed Result: All the color palettes appear white.

Note: Checked on unity version 2019.1.0a6, 2018.3.0b7.

Occurs In: 3.0.9, 3.0.0.

Environment: Win and Mac.