In this section, we will discuss the impact of color to the effect of hybrid.
The idea of hybriding RGB colorful images is just to treat RGB channels seperately. For each RGB channel, we build a laplacian pyramid. Then, we mix corresponding channels of two images. At last, we put the three channels back together.
At first let's compare the two hybrid images below. The left hybrid image is generated from two RGB images, while the right one is generated from the same image in gray scale (both are generated using the same algorithm and parameters). We can see that the right image looks more like a cat in large scale than the left image does. One reason is in the left image, the color of the dog(low frequency) doesn't quite fit the stroke of a cat(high frequency), so that it's not easy to recognize that as a cat. However, in the right image, the contradiction of stroke and background(low frequency) color is weaken by converting to gray scale, so that it's easier to see the cat.
If we convert the dog image into gray scale, and hybrid it with the RGB cat (the image below), we can see the result becomes much better, because we elminate the contradiction between background color(low frequency) and stroke by graying the background color (low frequency color).
However, when the low frequency color fits with the high frequence strokes well, whether to gray the background color doesn't make big difference. See the following example.
From the previous example, we can see that there is no big difference in whether using high frequency color. That makes sense because the color information is barely preserved in high frequency images. However, it's still better to not use color for high frequency images, because barely preserved doesn't mean not preserved.
For example, between the following two hybrid images, the left image still preserves some blue color, which looks a little bit strange.
For most cases, it's still better to use only gray scale images for hybriding images. However, when the strokes of high frequency image fit with the color of low frequency image very well-- which is usually not the case-- it's good to use color images for hybriding (because it looks beautiful!).