If they are things like trees or bears, you could still use them. As long as they are not extremely Christmas like and have Santa Claus or anything it should be fine.
Christmas and Halloween decorations are use to decorate our yards and homes for these particular Holidays. Halloween decorations consist of Scarecrows, Ghosts and Pumpkins. Christmas decorations consist of Nativity Scenes, Trees and Santas.
There are a lot of Halloween items that you can reuse for your Christmas decorations. For example, if you used candles as part of your Halloween festivities, these can add to a very tasteful Christmas setting.
If you are willing to forever change your decorations, you could cut and paint them. The paint will help transform them from Christmas to Halloween decorations. You could also use fabric as a temporary solution.
You could do a 'Nightmare Before Christmas' theme, like the movie, and use all your scary Halloween decorations for that. Or, instead of buying orange colors, buy black and white Halloween decorations and use the white ones for Christmas.
Although it may be difficult, Christmas decorations can be used to decorate the outside of your house for Halloween. An example of this would be white Christmas lights. These can be used to light up a collection of jackolanterns.
A good place to try trading some of your Christmas decorations for Halloween decorations would be Craigs List. You may also want to consider donating them to a store like Savers. They will give you a store credit that you can then use to purchase Christmas decorations.
Sometimes people use Christmas outdoor lighted decorations for Halloween because they are versatile, like the white lights. However, it is quite tacky to use decorations like reindeer/Santas so I advise against that.
The use of outdoor flag decorations make the option of incorporating Christmas and Halloween decorations into one device. Simply change the flag to appropriate Holiday decoration. Seasonal colored lights are another way to celebrate multiple holidays with the same decorations.
http://anna-marie-bowman.hubpages.com/hub/Decorating-Your-House-For-Halloween is a unique web site with wonderful tips and suggestions on how you can use your Christmas sale items for Halloween.
I'd highly recommend checking Youtube on how to reuse some of your Halloween decorations for Christmas. They usually have some great ideas and would also be able to help you learn how to make your own to add your own personal touch.
There are some places online that offer ideas on double-use ornaments. One site that I like is http://tlc.howstuffworks.com/family/halloween-decorations.htm.
Pink Christmas decorations might be slightly difficult to repurpose for Halloween but it can be done depending on what the decoration is. If you can add yellow coloring by dyeing or with a tint, the yellow will turn the pink to a shade of orange. If the decoration is something you can add black stripes to, that might work as well. Pink and black stripes are not unheard of for Halloween.