TF has done an Humbrol list. You should use the same colours as the shoulders if you're doing a SE Fett.
Shoulders/knees
- Halfords plastic primer, Grey
- 11 Metallic silver
- 74 Light yellow (around silver damage)
- 154 Yellow
- 82 Orange (added in tiny amounts to the yellow and over sprayed)
- 132 Orange/ red (added in tiny amounts to the yellow/orange and oversprayed)
- 79 Grey (added to the orange orange/ red to darken for weathered effect)
:cheers