Many creatives will convert images to CMYK first and then try to color correct images. The most common reason for this is because they want to see what colors will be lost when converting from the larger RGB gamut to the much smaller CMYK gamut, then make color adjustments to compensate for the colors that are impacted. Correcting images in the RGB color space will always yield the best results, mainly because you're working with a much larger gamut, and unlike the CMYK color space, you have not generated the black separation. Photoshop has a tool that shows an on-screen preview of what the image is going to look like when it's converted to CMYK while working in the RGB color space. Simply select View > Proof Colors. This gives you the best of both worlds: The image remains in the RGB color space with the full gamut of colors for correction, while an on-screen preview shows what the image will look like when it's converted to CMYK.