I did the simple "ben heck" composite mod on my Colecovision. 
http://benheck.com/Games/Coleco/Video_Mod.htmTbh, im only partially satisfied with it. The color/brightness balance seems a bit off. 
You adjust the strength of signal with a pot but it seems its never perfect. 
Too much signal and the image looks washed out, too little and it gets noisy/dark. 
There is no perfect middle ground, always some compromise it seems.
Its either dark image + noise or my reds look orange. 
Might be that my transistor is not perfect for the job. Im curious what others will say about it.