This tutorial was created by me, all images are screencapped from my computer by me.
Step 1: Open the image(s) that you are going to use for your colorbar. I usually use 100x100 size pictures. Create a new image that is the size of all of your images combined into a row (for example, with 6 100x100 images, your new image should be width 600 and height 100.)
Step 2: Convert your first image to greyscale.
Step 3: Increase the color depth of your greyscale image back to 16 million colors/24 bit.
Step 4: Create a new image the same size as your greyscaled image (in mine, 100x100) and flood-fill it with the first color for the colorbar.
Step 5: Paste this color block as a new layer into your greyscaled image (Ctrl+C, Ctrl+L)
Step 6: Open the layer palette if it is not already open and adjust the layer opacity of the color block layer. I typically set the opacity to around 33%.
Step 7: Merge the layers together
Step 8: Paste the merged image as a new selection (Ctrl+C, Ctrl+E) into the image created in Step 1 and move it all the way to the left edge of the image. Go to the Selections menu and choose "select none".
Step 9: Repeat steps 2-8 for each color block in your bar, matching the edges of each block.
Your finished colorbar should look something like this:
