hi
touche eclat is to be used alone. It isnt a good idea to use foundation under the eye because of the typye of chemicals and ingredents used in a foundation (as im sure you know, its much thicker and gives more coverage then a under eye product) the eye area is very sensitive and the skin is considerably thinner. same principal with using a conceler. If it conceler has anti-bactrial agents in it, it shoudnt be used under the eye.
just use a couple of dabs under the eye and pat around the area of coverage to blend into your natural skin tone. if you dont think its covering use a little more, dont use foundation, however tempting it is!
touche eclat comes in two shades 1 and 2. generally I offer shade 2. It is a universal shade that suits most skin tones. shade 1 is for extremly pale fair skin. It has a lot more pink tones in it unlike 2 which is more yellow.
The whole point in a touche eclat is to cover and illuminate and therfore its best to go for a shade lighter however my best advise is to go for 2 unless you are very very light blonde and very pale skin with more pink tones in your skin.
dont forget, you can always get a makeup session in a dpt store with an yyves saint lauraint consultant.
hope this helps!
x