According to the Methodist Church, is white grape juice an acceptable alternative in communion? A Methodist Church has changed from regular grape juice to white grape juice to keep stains from the carpet. Is this a valid Methodist traditional usage? Are there any historical sources explaining this usage?
Methodist Churches enjoy a great deal of flexibility when it comes to when and how they conduct their services, and this is illustrated by the freedom they have to use grape juice (red or white, it matters not) and bread that doesn’t contain gluten. Their attitude can be summed up thus:

In the essentials – unity
In non-essentials – liberty
In all things – charity

Below are links to a couple of articles about the history behind the use of grape juice within Methodist Churches:

In 1864, the General Conference of The Methodist Episcopal Church entered the conversation [about non-alcoholic grape juice] when they approved a report from the Temperance Committee that recommended “the pure juice of the grape be used in the celebration of the Lord's Supper.”
In 1869, he [Dr. Thomas B. Welch] perfected a juice pasteurization process in his kitchen and began selling “Dr. Welch’s Unfermented Wine” to churches preferring an alcohol-free substitute for Communion.  Unfortunately, the idea didn’t take off.  After four years, Welch gave up this side business.  Two years later, his son Charles convinced him to produce unfermented wine again. Charles offered free samples of the sacramental wine substitute to churches.

Source: http://www.umc.org/who-we-are/methodist-history-controversy-communion-and-welchs-grape-juice
See also: http://www.umc.org/what-we-believe/why-do-most-methodist-churches-serve-grape-juice-instead-of-wine
Removing stains from red wine or red grape juice is problematic.  And a large area of carpet could cost thousands of pounds/dollars to replace.  Prevention is always preferable to cure.