Yes, any decal you upload can then have it's color changed, so a black logo will work just fine.

Cuda, the gradient is actually even easier than that. I base painted the car in the blue middle color, then used the square gradient shape with the roof view to do two separate pieces, the green in the back and the dark blue in the front. Big enough square to cover the entire car, and setting the angle and depth to maximum actually covered the entire car at all angles. The tricky part is that the "Hood" on the 919 is considered to be the square-ish panel between the fenders and ahead of the windshield. I had to do a separate gradient square for that piece and just eyeballed it to line up the coloring with the rest of the bodywork. The "Other" on the car is also the two strakes on the opening in the side bodywork, but I just left them the base blue and it seems to work out.