This can be achieved by adding a background color in the bundle.yaml file for each panel in your tab.
this is an example from mine: (see the screenshot)
title:
en_us: BDJ-Dynamo
layout:
- 1 - menu
- 2 - BDJ2
- 3 - Controls
- 5 - Views
background:
title: '#4d74b78a'
# set background for all parts
#background: '#ff0000'
# or for individual parts
#background:
#panel: '#ff0000'
#title: '#00ff00'
#slideout: '#0000ff'
author: Bram de Jong (IDG)
you can either set a color for the entire background of the panel, or you can specify which color the Title needs to be (as Ive done in my toolbar), the panel itself and the slideout (if you have one)
Hope this helps and makes sense.
PS: I use https//rgb.to for colors.
Remember that adding ff in front of your HEX coloring gives you a non-transparant color, while adding 4d in front, it gives a slightly transparant color.