Which planet is described as orange (pluto, saturn, mercury, uranus) ?

Although it's not on your list, I think Mars would have best fitted that description, but from the four options you give, I'd say, from all the illustrations of planets I've seen, Mercury would be the one, though it is usually pictured as being rather more yellow than orange....

Mark - Your quite correct - The DK book of Stars and Planets describes Mars 'easy to spot with the naked eye at or near times of of opposition, when it resembles a bright and distinctly orange-coloured star.

Of the four mentioned, I have found Saturn described as 'browny orange.'

Saturn has rings around it. If you could see it in space it would look like a browny orange circle with rings around it. The rings are made of ice and dust. It has 18 moons. Saturn has storms on it. Saturn is an outer planet.

https://www.nasa.gov/mission_pages/cassini/media/cassini-072104.html

Mars is the bright,yellow-orange beacon low in the south eastern sky around midnight.
With a small telescope,the planet will look distinctly orange-red. The Martian soil is chock full of iron oxide,otherwise known as rust.
https://www.space.com/mars-skywatching-at-opposition-fall-2020

Saturn is yellow - Uranus is green.
Mercury resembles the Moon in appearance - Pluto is so distant it can hardly be seen with a telescope.

Stars and Planets Handbook by Ian Redpath and all websites point to Mars, so ranu what is the answer,eh?