Emissive Trims Shaders is a core shader. It makes armor trims emit a bright glow without OptiFine or Iris. In vanilla Minecraft, core shaders are loaded through the resource pack system, so you turn it on from the Resource Packs menu rather than a shader menu.
Enable the pack
1. From the title screen or pause menu, open Options → Resource Packs.
2. Find Emissive Trims Shaders in the Available list on the left.
3. Click it to move it to the Selected list on the right.
4. Keep it near the top of the Selected list. Packs higher in the list override packs below them. If another pack also changes armor or shaders, putting this one above it gives the glow priority.
5.
Test the glow
1. Put on a piece of armor that already has a trim applied. Untrimmed armor has nothing to light up.
2. Go somewhere dark, such as a sealed room, a deep cave or the surface at night.
3. Press F5 to switch to third-person view so you can see your own armor.
4. The trim pattern should shine brightly while the rest of the armor keeps its normal vanilla look.
The effect stands out most in low light, so a brightly lit base is a poor place to judge it. If nothing glows, check the pack's position in the list and that your armor actually carries a trim.