The GUI creates a "shadow" based on the light of the main dimension and light color when viewing a portal with Immersive Portal and Optifine active. Other versions also render only the portal it leads to.

Mod:

• Immersive Portals 0.24
• Optifine preview 13

Shader: BeyondBelief Vanilla Reborn

Shader: Chocapic13_V8_Ultra

Shader: SEUS Renewed 1.0.1

And, as a bonus, a path-tracing shader pack called MollyVXE

Fixed in 0.25