This is an issue about the channels panel, looks like a bug to me.
In 1.6, selecting a single colour, with alpha deselected, shows a B/W image, presumably of the colour channel. Same with 1.7 beta. However, adding alpha gives a monochrome image of the proper colour in 1.6, but a fully coloured image in 1.7 beta. If I add another colour instead of alpha, in 1.6 I get an image with the proper color mix, in 1.7 beta I again get a fully coloured image.
I have reproduced this at will with any .jpg I tried. For some reason, uploading a file failed with -200, so no example, but I do not think it makes a difference.