Why if I load some black&white on aperture, it autoatically tranforms then in colour photos?
Aperture does not transform any image files on import. It stores the original files and renders versions, to display the original.
Are you shooting raw, and have you set in-camera settings to turn your photos into Black&White? The raw-files do contain the original color images, and only the embedded jpeg will show your Black&White processing done in the camera. So when you import the raw to Aperture, you get the original image from the sensor, not the processing done in the camera - that is the purpose of importing raw. You will see the the embedded b&w jpeg for a short moment, while Aperture developes the raw.
If you want to import images with in-camera processing, don't shoot raw, but jpeg. Then the file that Aperture imports, will have all your in-camera processing.