Playing with the colors in 1555 mode

In a ELEMENT_STATIC_IMAGE, I would like to display a grey area with a variable level of grey.I tried to do that by modifying the image content through image_data.final. First I read the format, which is always IMAGE_15_BIT (1555), then I change to bytes of the image to set the red, green and blue values (each of them is on 5 bits) as required to obtain the desired level of grey.But it doesn't work. I get very strange results, strange color variations instead of levels of grey. Does it mean FS transform the 1555 format into its own color-table model? If so, it means I can not do what I want here... Any idea?Thank you,Eric