How did the artist plan this portrait?

NPG 6353

Infrared reflectogram showing underdrawing

Using infrared reflectrography we can see that the artist made a drawn design before starting this painting. The drawing is quite free, delicate and skilful, indicating that the portrait was carefully planned and included most of the key facial features. The underdrawing carefully details the main elements of the composition, such as the shapes of the eyes of Flicke and Strangwish. The artist also laid in some of the details of the costume.

To ensure the inscription was perfectly level Flicke ruled lines as a guide. These can be seen in the infrared photograph below.

NPG 6353

Detail of side of Strangwish's face - showing underdrawing visible beneath flesh

NPG 6353

Infrared reflectogram showing inscription ruling

What are the colours on this artist's palette?