Wow that's crazy. Could it be a reaction between your current powder/foundation and the Guerlain bronzer? It might be accelerating oxidisation of the bronzerI have not tried Sleek but I have tried the Guerlain Terracota 4 seasons. I am NC 50 but the colors go on smooth and very pretty but it seems to get darker on my face as the day goes on.
Even though I use a fluffy brush, I am thinking its too dark /red for me, whether I mix all the colors or just the 3 browns. #8 Ebony does the same, it darkens as the day goes on, a lot. I also ordered #5 bot I am on the fence about it, it does not darken but it comes out as a glow and I wear it with margin. It is not very dark & it is almost impossible for me to put too much on, I use an ELF blush brush.
I am debating on which to send back, keep or exchange.#7 also darken & became very red/ brown even though I used a fluffy brush (MAC 138).
Do you use it just to contour like the perimeter of your face or as an all-over bronzer?