Thank you, pkchan, tangxiao and orientalhorizon, and all commenters.
After some investigation, I think both the answers by pk and tangxiao are equally acceptable. 色度抠像, the combination of two classic terms 色度 and 抠像 is undeniably right, but 色度键 is also a term widely used today. BTW, another form similar to the latter, 色键, seems to be used by more people today.
I am sorry I have to pick one to give the points from the two equally helpful answerers, but my gratitude equally goes to both of you.
As to 影像叠加, the translation suggested by orientalhorizon, I think it is the Chinese for "image overlay", a collective term including "chroma key" and peers. Please read the following URL provided by tangxiao for details:
http://blog.video.com.cn/faq/archives/2005/2479.shtml
The translation workplace 
peer agreement (net): +4
