Can your re-verify your age. For a 15 year old, these questions are a bit pre-mature. You look more matured the way you are looking at things. Like today's Yuppy generation X you should be asking about what mods can be done to Liana or how much speed can it attain.
Anyway, coming back to your questions. there are very attractive colors available. I have the detail in my office. Will post them tommorrow. What I remember...white, silver, black, Navy blue...rest later.
As regards CNG cylinder...brace yourself for this.....there are 2 SLIM cylinders, thereby offering a lot of space then the conventional single cylinder.
Comparison with XLi City. Well, each one has its pros and cons. I presume you chalk out 10-20 major attributes that you feel should be present in a good car. Draw a chart for the three cars and see which one tops. I could help you prepare that chart for you because I am also facing the same dilemma. Remind me after one week because by then the CNG version Liana would have arrived at the showrooms.