The color of mucus when you have the flu is usually yellow and then turns to clear when you are getting over the flu virus.
When I have the flu and am spitting up and coughing up mucus or phlegm the color of my mucus or phlegm when I have the flu is Yellow.
As your flu goes away the color of the mucus may change to a clear mucus.
It's also normal to sometimes have some blood in you mucus that you're coughing up as that is just a sign of the infection of the flu virus.
As long as you're not coughing up huge amounts of blood then there's nothing to worry about with just a little bit of blood in your mucus.
You might also have a bit of blood when you blow your nose as well which is also normal.