If you want to make the base texture lighter, download GCFscape and VTFedit if not already. Open the TF2 .VPK files tf2_misc_dir.VPK and tf2_textures_dir.VPK, found in the /tf folder, using GCFscape. Click the Find button (looks like a magnifying glass) and with the settings 'Match: Substring' and 'Case Sensitive: Not checked' search for your texture name (In your case it is harbor_sign002). Within the tf2_misc_dir.VPK file you should find harbor_sign002.VMT and within tf2_textures_dir.VPK, harbor_sign002.VTF. Extract both of these to a folder anywhere by dragging them from the GCFscape window to the folder, or clicking 'extract' and choosing a location. Rename the files to something like harbor_sign002b, and open up harbor_sign002b(or whatever the name is).VMT with a text editor or VTFedit. Change the line that says `"$baseTexture" "signs/harbor_sign002"` to ` "$baseTexture" "signs/harbor_sign002b"`and save - this is per name, whatever you named your VTF must be what is written for baseTexture. Next, open up your VTF file, and press ctrl+e to export it as an image file, usually it is a PNG if I remember correctly. Open it up in your favourite image editor, and simply change the brightness of the image, I'll not go over that here. Save the file, either as a PNG or TGA, and then go back to VTFedit. Press ctrl+i to import a texture, and select your newly saved texture. Hit ctrl+s and overwrite the existing harbor_sign002b.VTF you have saved. Now, copy both the harbor_sign002b.VMT and harbor_sign002b.VTF files, and paste them into your team fortress 2 folder with the following directory: /tf/materials/signs/ - you can find this directory by looking at the VMT's basetexture line. If one of these folders doesn't exist, just create it. Now, close the Hammer editor if it was already open, and open your map. The texture should be available to you in the material editor. If it's too dark or too bright, just keep playing with the brightness in your image editor, before saving and re-importing the image with VTFedit. Don't forget to overwrite the existing .VTF file in materials/signs/, otherwise your changes won't appear.
Hopefully this helps you