White slipcovers are easy to keep clean - just whip it off and toss it in the washer
(preferably a big commercial washer) or send to the cleaners.

The problem is that brown and green don't lend themselves to coastal - Theyre more "woodsy". When I think coastal I think sand beige, sky blue, Navy, coral pink, light green, white, linen, cotton...
...I can't think of anyone who enjoys coming in from the beach to sit on a brown leather sofa - sand, sunburns & leather dont mix.

Thanks! I know it doesn't mix at all. I need to get a new slipcover sofa, but this leather sofa is only 2 yrs. old from Ethan Allen, and I will never recoup the $$ I spent on it on Killer Craigs List. :) Nor from a consignment store. I guess I'll just have to cut my losses and consider this a learning experience.
We were going to take it upstairs to the bonus room, but the legs don't even come off, so it won't fit through the doorway.

It would be great if you could show some pics of what you have. I totally get wanted to keep the piece and trying to work with it as best you can. You may not achieve a complete costal look, but might get close to what you like. I would use coastal colors and materials in pillows and a lightweight throw on the couch- and then add some accessories to your surroundings to downplay the leather seating. If you really want to cover the leather and dont want to use white, try a natural linen in shades of sand or even grey!

Well, let's look on the bright side - the leather is easy to keep clean, sand just wipes off. First of all, maybe the "generic" duck cotton slipcover was just ill-fitting? I've never had luck with them if they weren't custom made, so you can do that if your budget allows. An Ethan Allen sofa is an investment, so I understand that you want to hold on to it to get your money' s worth, and there's nothing wrong with that. Can you change the green slipcover on the loveseat? Maybe to a tan and white wide stripe OR a tea-stained pattern with yellow, cream and brown, to at least tie in the brown couch that, let's face it, is going to dominate the room. Get a cream coloured rug with a little brown in it, along with the other colours you end up with on the other pieces, or a sisal rug. Place some big wicker baskets by the ends of the sofa or a rattan sofa table behind it that hides it if it's floating in your room, with a lamp and other accessories (sailboat) behind it, that will get peoples eyes go to that rather than the sofa. Get some striped pillows in cream and your chosen colours and there are even woven raffia-type pillows that you can put on your couch. Also, the ubiquitous throw in white or cream. In other words, don't do ALL these things, pick the colour and look/feel you prefer most or is most feasible and go with it throughout. As savanza said, if you could provide a pic, it would really help!

Thanks everyone! I received my cream color slipcover Saturday, to replace the green one. I have a seagrass area rug and we're getting ready to put built ins on either side of the fireplace, as well as paint the walls blue. My pillows and throws and accessories are blue, green and white/off white.
I will try to provide a pic.
And you're right about the leather being easy to keep clean, opposed to the ill-fitting slipcover....we have 3 dogs. :D

Here are pics with and without the slipcover on the brown sofa.
The green slipcover is really a much lighter green than it appears in the photo.

Can you show the pictures of the completed room?

heypam, if the linen slipcover isn't working for you, here is an inspiration room that pulls off a coastal/cottage look with a leather sofa. This room is successful because it has texture, liberal use of white and fresh color.