Unable to open a colored image layer

If your image is in grayscale or color-indexed mode (see the title bar),

anything you add to it (cut/paste, or Open as layers) is coerced into its current mode (for grayscale: desaturated, for color-indexed: converted to the closest colors in the colormap).

You can restore full color support in your image by putting in it "RGB" mode: Image>Mode>RGB

If you did not set the mode explicitly, Gimp opens an image in a mode that is coherent with the source format: GIF (and some PNG) are color-indexed, some not so frequent JPEG and PNG can also be opened as grayscale images (no color information).