Creating NSImage from NSColor

A simple category method will do this

@interface NSImage (ImageAdditions)

+(NSImage *)swatchWithColor:(NSColor *)color size:(NSSize)size;


@end

@implementation NSImage (ImageAdditions)

+(NSImage *)swatchWithColor:(NSColor *)color size:(NSSize)size
{
    NSImage *image = [[[NSImage alloc] initWithSize:size] autorelease];
    [image lockFocus];
    [color drawSwatchInRect:NSMakeRect(0, 0, size.width, size.height)];
    [image unlockFocus];
   return image;    
}

@end

[EDIT] remove deprecated API

If you're using AppKit, here's a Swift 5 convenience initializer version of the other answers. Sadly, this doesn't work in UIKit 😞

extension NSImage {
    convenience init(color: NSColor, size: NSSize) {
        self.init(size: size)
        lockFocus()
        color.drawSwatch(in: NSRect(origin: .zero, size: size))
        unlockFocus()
    }
}

Usage example:

let redSwatchImage = NSImage(color: .red, size: NSSize(width: 128, height: 128))

Feel free to change the semantics as necessary 😁