This is just a quick tip for those of you who have just opened a new file in Photoshop that you wish to make changes to but have found that all it contains is an index layer. I recently did this myself and tried for a while to unlock it just so I could add just a simple transparency gradient to it. After much research I found out how to do it. I will also add how to unlock a background layer while I’m at it cause I’m a nice guy.
Unlocking an index layer in Photoshop CS5.5:
Select the index layer from your layers palette then select from the top menu, Image >> Mode >> RGB color… Your index layer is now unlocked and may be edited to your hearts content.
Unlocking a background layer in Photoshop CS5.5:
Right click on your background layer. Now in the menu that appears you have 2 selections that will do the trick. Layer from Background will unlock the layer for editing. This is not advised as your are working with the original. The better option is Duplicate Layer. This way you still have the original even if you hide it and can work with a copy. The background layer itself will remain locked so you cant accidentally modify it in any way.