Terrain details colors not working properly

When painting details on a terrain, the colors aren't being painted over the textures properly unless billboard is checked.

For instance, I've got a grass texture that is originally a very light gray. If I check "billboard" when creating a brush and pick dark green as the grass color, the grass is gonna be dark green. If I don't check "billboard", the grass is gonna be almost white, with a slight tint of green.

This wasn't the case in a previous version, so I might have changed something by mistake while I was doing other things. It happens even if I do it on a different terrain, or a different scene.

Anyone knows why?