The GIF format was introduced in 1987 and contains one eight-bit channel of data giving it the ability to reproduce up to 256 colours at any one time from a palette of 16.8 million. This means that although a GIF is no good for saving continuous tone imagery such as photographs, it is extremely well suited to images that contain large blocks of colour such as logos and line art. The advantage of saving so few colours is found in the sharpness of detail and the ability to keep file sizes low. This makes it a great format for web delivery and as such has been utilized by web developers and designers around the world.
The reason I'm writing this article isn't to go through the technical process of optimising and saving to the GIF format, if that's what you're after then my 15+ minute video tutorial entitled 'Saving a GIF' from my series on File Formats is worth a look. The focus here will be on how to open and edit a GIF as it's a popular question and sticking point in many Forums, and something that isn't always easy to understand. So here goes.
The image below is something designed to get us all into the spirit of Valentine's Day. If you want to follow along simply right click on the image and choose the 'save as' command. I've also illustrated how the Layers Panel is going to look once you've got the image open in Photoshop, just one solitary layer called 'index'. Notice that, unlike a standard background layer, we aren't able to unlock it by double clicking. Nor are we able to get anywhere by right clicking as all the options are unavailable. And if we go to the Layers menu it's a similar story - everything dimmed down to indicate no access to the options. In fact, the more we look around, the more we notice that features are locked up, unavailable or not even there. We can't add layer effects, styles, filters, and even some of the colour adjustments are closed for business. So what gives?
The main issue here is the inner workings of the GIF file. As already discussed, it may support the 16.8 million colours of an 8-bit per channel RGB image but only a maximum of 256 at any one time. So instead of operating in an 8 bit-per-channel colour space, it's able to use a colour look-up table to index the colours and operate within an indexed environment. So Photoshop opened the GIF inside its indexed colour space instead of its RGB one, and as you can imagine, the indexed colour space is far more basic and isn't supported by all the options and features that we can't get working. In order to get them working we need to switch from an indexed space to an RGB one, here's how we do it-
Come up to the Image menu and select the Mode option. You'll notice a tick next to the Indexed option acknowledging that it's currently active. All we need to do is select the RGB Colour option and we're now working inside an RGB colour space. You should notice the locked index layer transforms to a locked background layer, and all the options that were previously dimmed are available again.
If you're planning to save back to a GIF once you're done editing keep in mind that you may need a different set of indexed colours to make sure the file is saved as efficiently as possible. Remember, if you need more info on that, check out my 'Saving a GIF' video tutorial here.
I hope that explains how to successfully open and edit a GIF file in Photoshop, and why it's necessary to switch colour modes to unleash the full editing capability of Photoshop.