If it's just plastic, you'll never get a black mark. To get color on plastic, you need to mask and color fill (or just color fill if you are less sloppy than I).
The cool color changes you see on other gear is due to the painted or anodized metal case. To get black on unpainted and unanodized metals, you will need Cermark or something like it. There are now colored versions of Cermark out but I've not tried them.
70W Epilog Legend 24, bought it new in December 2002