You can work in garter. (There are rugs and blankets, with colored borders, colored blocks, done in intarsia that are garter.) You'll still need to wrap the old/new yarns at the color change. Just make sure to mark which side you'll do your wraps on (making that your WS).
You'll work the border in color A, twist it with color B to work the middle, twist the B with another skein of A to work the border on the opposite end. Very basic.
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re: garter intarsia.
of garter intarsia.