Greyscale vs 'RGB Grey'

Introduction

FIGURE 1  Black & White images can be either in a traditional single channel Greyscale, or as three channel 'RGB Grey'

View any Greyscale image in Photoshop and regardless of whether it is a single channel Greyscale, or has been converted to a three channel RGB, then it will look the same.  This is because Photoshop is colour managed and knows how to present the two different formats.  Converting between the two is straight forward by selecting IMage > Mode > Greyscale, or Image > Mode > RGB Color from the menu bar.

So if they look the same, then why consider turning a Greyscale image into a three channel 'RGB Grey'?  The answer is that many uses for Greyscale images are not done using colour management.  PowerPoint slide shows and websites are the most notable.  The time will come when colour management is used by everyone viewing a website, but that time is likely to be some way off.

NOTE:  An 'RGB Grey' image may technically be a colour image, but it will still appear grey as the three RGB channels will be identical.

Greyscale Images on Websites

FIGURE 2  A typical RGB colour image

Here is an RGB image which for demonstration purposes will be converted into a single channel Greyscale image.

FIGURE 3  The RGB colour image converted to a single channel Greyscale

Here is the result.  Other than converting to Greyscale, no extra work has been done.  Do you see a problem?

FIGURE 4  The single channel Greyscale image converted back to RGB, but now as an 'RGB Grey'

Now here is the same Greyscale image, but this time converted back to a three channel RGB image.  Of course the original colour has not been recovered as that was thrown away in the first conversion to Greyscale.  Again, no other work has been done to the image. 

Do you see a difference between the two versions above?  Website browser software is not normally colour managed with the result that single channel Greyscale images will have a collapse in the shadow detail and look far too dark.  Some newer versions of web browsers do now come with colour management options, but most users will never know about this and so will not have it switched on.  This can be solved by ALWAYS having your Black & White images on a website in the RGB Grey format.  There will be an increase in the image file size, but the disadvantage of this will be far out-weighed by the benefits of viewing the images with decent shadow details.

The same goes for PowerPoint slides:  ALWAYS use the 'RGB Grey' format for any Black & White images you use in the slide show.

NOTE:  If the two grey images above look the same and the first does't have a collapse in the shadow detail, then it is likely that you are viewing this website with a colour managed web browser.  If you have your own website then it is best to have your browser colour management switched off.  Why?  Because it is best to view the website the way your customers and clients will view the website.  If you see a problem with the images, then you know that they are seeing the problem too and you are now in a possition to fix it.

Conversion Warnings

FIGURE 5  Some images may appear to be grey, but are in fact RGB colour images

Converting Greyscale images to 'RGB Grey' is simple enough, but if you ever have to convert back again to Greyscale then it may be more complicated if the 'RGB Grey' images are mixed in with RGB colour images as you run the risk of throwing away the colour from images that may appear grey at first sight, but really are RGB colour images.

FIGURE 6  The Histogram shows that the above image is colour as the three RGB colour channels are not identical

If in doubt, then take a quick look at the Histogram.  A true 'RGB Grey' image would have identical Red, Green and Blue channels.  The Histogram above shows that the image is indeed a colour image.  This may seem obvious to you when looking at just one example here, but if you have been working at your images all day then you will start to get "blind" to the differences.

NOTE:  If you are facing the problem of having to convert many Greyscale images into 'RGB Grey', or back again, then there is JavaScript available that will automate the job for you.

There are also issues with colour spaces, for RGB colour images, when used on websites.  See the page on 'Website Colour Profile Errors'.

Quick Summary

RULE #1:  When supplying images to clients, always check to see what their standards are.  Many picture libraries are now asking for Black & White images to be supplied as 'RGB Grey'.  Publishers, though, will usually require them as Greyscale.

RULE #2:  Always use 'RGB Grey' for Black & White images in PowerPoint slide shows.

RULE #3:  Always use 'RGB Grey' for Black & White images for use on a website.  Be prepared for an argument with the website manager, if you have one, as they will often complain that this means an increase in file size.  But don't let them change your mind on this!  You need to sell your images and so the viewing quality is far more important than the issue of a modist increase in file size.

RULE #4:  If you are a designer preparing images for a print publication then do check that all your Black & White images are in the single channel Greyscale format.  A Greyscale image will print in Black, but an 'RGB Grey' image will be converted into a four colour image using all four of the printing CMYK colour inks instead.  This would be a workflow problem if the page was planned to be printed in Black only, or a cost problem if colour was added to the publication where none was intended.  Worse still, if you assume that the grey images are already Greyscale, without checking first, then you will be sending RGB images to the RIP device which will result in some real problems. Unless, of course you have set up your PDF workflow correctly where this problem would never happen.  The designer is responsible for the colour conversion to CMYK, so don't go and blame the photographer in this case.