When I was in the depths of my depression last year, colour is what I'd lost. Well, what I had actually lost was hope, but how did this loss of hope translate? My landscape had gone from full colour to black and white. In the midst of my depression, my life as a landscape (the one in my head, you see) looked dull and grey, as far into the horizon as I could see. This, in turn, led to a further loss of hope and a further depression.
I thought of all this when I got home that night, and it led me to conclude that "stay colourful" is a very profound thing to say. It's a very good thing to remember. When you're feeling sad, do your best to stay colourful. Dress in colour. Eat colourful foods. Notice the colours in your external landscape so that your inner one doesn't fade into greyscale.