Wish you were closer....I have a load of leftover B&Q Grasshopper green I used to zing up one wall. Gorgeous.
Go to Homebase.....find a sympathetic looking man....open your eyes wide and explain your problem....hinting...just hinting....."Too late to exchange, I suppose?" Look sad.
If that doesn't work ask him to make suggestions of how you can overcome this dilemma because you are clueless about mixing paints.
Yes I know it's playing the helpless woman! And why not? x