The purple of the ultraviolet light is distorting the red (which is at the opposite end of the color spectrum.) If it's turning orange try to shine a red LED on it, that may make it look more red. If it's turning pink, try an orange or red LED, that may make it look more red. Lighting is tricky stuff, you have to balance the red/ blue/ green (the three primary colors for light). Its not like mixing paint, where red and green mix to form brown; in lighting: red and green mix to form yellow. The mixing of colored light is called "additive" mixing, as opposed to "subtractive" adding of colors when mixing paint. So you might want to experiment with different colored LED's or cold cathodes. You might also just try bathing it in lots of red light, but that might make it more pink, I'm not sure.