Best How To :

For the launch .xib you won't be able to change the color programmatically. You can input hex values in the side menu. Click on background color, choose the second object from the left, and you can input hex values along with RGB values.

User the color hex website to convert your colors into RGB values if you want to use the color throughout your app (when you can do things programmatically).

If you have specific and consistent color needs, I would recommend creating custom categories to store your custom colors. An example answer is here.

UIColor+CustomColorCatagory.h

 #import <UIKit/UIKit.h>

@interface UIColor (CustomColorCatagory)   //This line is one of the most important ones - it tells the complier your extending the normal set of methods on UIColor
+ (UIColor *)customColor;

@end

UIColor+CustomColorCatagory.m

#import "UIColor+CustomColorCatagory.h"

@implementation UIColor (CustomColorCatagory)
+ (UIColor *)customColor {
    return [UIColor colorWithRed:0.643 green:0.643 blue:0.643 alpha:1];
}
@end

Hope this helps!