I wanted to give my Commander a bit of flair, so I gave him a shield and the base shield/fire effect from the Twin Lance kit. Here's some of the details on how I painted that.

Could probably be used for shield generators from other factions too, though you may want to leave out the base effect piece.

The shield was inspired by u/Additional_Count_801's Riptide here: https://www.reddit.com/r/Tau40K/comments/1wmrd5m/red_means_fast_2/

Shield Generator

Undercoat and paint the shield as per usual. My Tau are from the Ke'lshan sept, so it's a mainly grey paint scheme, undercoated in Mechanicus Standard Grey . See here for more details: https://paintpad.app/recipes/46237-kelshan-battlesuits.

Glow Effect

1. Drybrush around all active parts of the shield generator with a mix of Sotek Green / Temple Guard Blue . Be a bit generous on this step
2. Once that is dry, drybrush again, a bit less, with Temple Guard Blue . Leave the previous Sotek/Temple mix visible on the outside.
3. Drybrush again with Baharroth Blue , leaving the previous steps visible further away from the active generator.
4. One final drybrush with a mix of Baharroth Blue / White Scar , staying close to the active generator.

Light Source & Highlights

1. Paint all active sources of light with White Scar . This may take several coats.
2. Highlight edges immediately beside active light sources with a mix of Baharroth Blue / White Scar

Lightning Arcs

1. Paint lightning bolts connecting active parts of the generator with a mix of Baharroth Blue / White Scar . Be sure to get down into cracks in the panels.
2. Highlight the middle of the bolts with White Scar

Shield Bubble

1. Undercoat with Grey Seer
2. Base coat with Sotek Green . (I started with Stegadon Scale Green but that can probably be skipped)
3. Layer the whole shield bubble with a mix of Sotek Green / Temple Guard Blue shading to pure Temple Guard Blue at the top. I attempted to wet blend this, with middling success.
4. Line the cracks between shield 'panels' with a mix of Sotek Green / Contrast Medium
5. Highlight the panel edges with Baharroth Blue

Flames

1. Undercoat with Grey Seer
2. Base coat with Yriel Yellow
3. Layer with Averland Sunset , leaving the previous layer visible at the base of the flames.
4. Layer again with Troll Slayer Orange further up the flames (I used Fire Dragon Bright as that's all I had, but Troll Slayer is a better, deeper colour for this).
5. Continue layering the top in successively smaller amounts, next with Evil Sunz Scarlet , then Mephiston Red and finally Khorne Red
6. Highlight the very tops of the larger flames with Abaddon Black