Yes, you can mix colors. Since you are using Semco for the first time, realize that regardless of the number of coats (and color selection) you are not going to get a sheen or gloss similar to that of varnish or Cetol. I have used Semco for a number of years and currently recoating some teak now. The primary reasons that I use Semco is that it does a good job in protecting teak and the ease in preparation and application. I wouldn't go so far in saying that it is an attractive finish. BTW, I usually use the "natural" color because It is a bit more translucent and allows the teak's natural color & grain to be seen.