Replace Galacticraft's default dark airlock with connected textures that match the Tin Wall Blocks. The seal of the airlock is red.

An Airlock with Tin Wall Block based connected textures

Tin Airlock

The ConnectedTexturesMod (CTM) is required for this resource pack to work.

You might also be interested in my Connected Textures for Galacticraft resource pack, which adds connected textures for other Galacticraft Blocks such as all of the blocks based on the Tin Wall/Decoration Blocks including Sealable Pipes and Cables.

If you do want to use this resource pack together with the Connected Textures for Galacticraft resource pack, then the Tin Airlocks resource pack should be above Galacticraft CTM as shown on the right side of the picture below.

Tin Airlocks and Connected Textures for Galacticraft load order

Resource Pack Load Order

You may wish to instead install a combined resource pack such as the one which can be seen on the left side of the picture above. The combined resource pack can be downloaded from the Additional Files section of the corresponding version of Connected Textures for Galacticraft (v1.1.0 onwards) or using the link below.

Galacticraft CTM + Tin Airlocks - v1.1.0