Both light blue and white would look very sharp with black slacks and a tan jacket, but you could just as easily go with something that pops more too. You've made a good choice in using two neutral colors, so really the sky is the limit. I do think - and this is only my personal preference - that I would avoid wearing an exact match to either the slacks or jacket though. Some people can carry this off so it is not something you should worry about if you would really like to wear a tan or black shirt. But if you would like to go that route, maybe a darker brown shirt, or even a medium gray to charcoal shirt would look good and not quite as innocuous. The thing is, you've done very well with the basics and now you can allow yourself a lot of leeway. It would be very hard to go wrong. You could even get away with a design in your shirt if you'd like. One of my favorites with the colors you've already chosen is a dark gold or olive green shirt. That always looks very classy too.
Well I would wear a white shirt because whit goes with any thing!
In my opinion I wouldn't mix colors.I will say Black shirt or a creme tone,but nothing really colorful because the tan jacket that you're going to wear ,if you wouldn't wear any jacket it's more options about mixing colors.I hope I help.
Since, black is neutral, you can try the classy colors like classy blue- lavender with a satin tin or any other color because they like all like up to look awesome!