Setting the Color of the Curve (IM: XOv)

Inside Macintosh: QuickDraw GX Programmer's Overview / Part 1 - Getting Started With QuickDraw GX: Chapter 2 - A Quick & Easy Sample Program

Setting the Color of the Curve
Another property of your curve shape object is the reference to its ink object. One of the properties of the ink object is the color property, which contains a data structure that specifies the color that QuickDraw GX should use when drawing the curve.
This structure is defined by the gxColor data type, which contains three fields:

The space field specifies what color space your color is specified in (such as grayspace, RGB, or CMYK).
The profile field specifies color-matching information that QuickDraw GX uses when converting device-independent colors to color-corrected values on a particular output device.
The element field is a union specifying the color in terms of the color space. For instance, if you are specifying an RGB color, this field contains color values specifying the red, green, and blue components of the color. Colors in the grayspace color space have a single color component, representing the luminance (from black to white) of the color.

Color components range from 0 (no intensity) to 0xFFFF (maximum intensity). To set your curve's color to a medium gray, you can use this sample color structure definition and this setting for the color structure's fields:
gxColor halfGray;
halfGray.space = gxGraySpace;   /* color space is grayspace */
halfGray.profile = nil;         /* no color-matching */
halfGray.element.gray = 0x8000; /* 50% intensity */
Once you have created a color structure, call the GXSetShapeColor function to copy its information into the color property of the ink object referenced by your curve shape:
GXSetShapeColor(aCurveShape, &halfGray);
The GXSetShapeColor function works analogously to the GXSetShapePen function, it

finds the ink object referenced by your curve shape's ink property
copies color information from the halfGray color structure into the color property of the ink object

This function affects only the color property of the ink object; no other properties are affected and there are no visible results until you draw the curve.