The Great Ghost Rescue by Eva Ibbotson, published in 1975.
Humphrey the Horrible is a pleasant, friendly ghost - quite unlike his frightful, ghastly and loathsome family: his mother, a Hag; his father, a Scottish ghost killed fighting in the Battle of Otterburn in which he lost both his legs, and was run through by a sword; his brother George, a screaming skull; and his sister, Winifred, a wailing ghost covered in bloodstains.
The ghost family are turned out of their castle home when humans plan to redevelop the castle into a holiday resort. They travel across England, accompanied by their headless Aunt Hortensia and their pet Shuk, and come to Norton Castle School, mistaking it for an empty castle. Here, they meet Rick, a student quite unafraid of ghosts. Rick plans to take the ghosts to the Prime Minister for peace talks concerning the large numbers of ghosts being turned out of their homes.
The ghosts and Rick head to London, and pick up an assortment of hangers-on along the way: Walter the Wet, a ghost haunting a polluted river; Cousin Susie and her vampire bat brood; and the Mad Monk, whose church was destroyed to make way for a motorway.
In London, Rick seeks out his member of parliament, Clarence Wilks, but the ignorant politician dismisses Rick's story as a fanciful pretense. Rick and the ghosts, furious at Wilks' disbelief, haunt his house and ruin a dinner party with several prominent guests. Wilks takes the ghosts to meet with the Prime Minister in exchange for leaving him be.
As you can see on the image, Humphrey is pink. The family is losing their home to developers. They are helped by a human. There is a ghost that turns into a bat. His sister constantly seeks to wash the stains from her nightgown. And this review indicates that there are illustrations:
The Great Ghost Rescue is great for young readers who have made the transition to chapter books but still need a relatively simple plot and minimal drawings (there are a number of black and white/pen & ink drawings to bring some additional life to the story...heh).
It looks like the pink part was omitted for the 2011 film release: