So, just to clarify.
Do you need to rasterize PDF page or you would simply like to extract embedded images in PDF?
a) If you want to rasterize the entire page you would use PDFDraw (PDFRasterizer is just more low-level & I wouldn't recommend using PDFRasterizer unless you have a good reason). You can rasterize to RGB, CMYK, or Gray color space. Typically most users rasterize to RGB (which is default), but if you can change this using PixelFormat in the call to PDFDraw.GetBitmap() or PDFDraw.Export(). When you rasterize in CMYK, PDFDraw would use CMYK for all pixel blending operation and would preserve black channel (and so forth) which is important in many print workflows.
Please note that you can control conversion from CMYK to RGB (and the other way around) using PDFNet.SetColorManagement(PDFNet.CMSType.e_lcms) & PDFNet.SetDefaultDeviceCMYKProfile("cmyk.icc") & PDFNet.SetDefaultDeviceRGBProfile("rgb.icc") (see comments at the beginning of PDFDraw sample). By using different profiles, you may end up with very different color output. Btw. if all original images are RGB and am not sure why you would rasterize in CMYK?
b) If you want to extract embedded images in PDF, you should use approach shown in ImageExtract (