The RGB image can be transformed into YCbCr domain which Y channel can be treated as a gray image.
Then, you can use fusion method to fuse Y channel of RGB image and single band image.
Finally, you add the color information(Cr and Cb channel) into the fused image.

Thank you for yous suggestion! So isn't there a direct way to fuse the RGB image and single band image?
I try to transform the RGB image to the IHS domain firstly and then use the I channel to fuse. The fusion result looks ok, but I cannot find the codes of some evaluation indexes, like SAM, CC, ERGAS and QNR. It's so kind of you if you can provide some codes of indexes to me. Thanks!