Based on your primary color of #0077CC and the requirement for a striking secondary color for CTAs along with modern cold greyscale tones, here is a suggested color palette:

 <Modern Cold Greyscale with Striking CTA>
1. Primary Blue - 
                            
                            #0077CC
                         - A vibrant blue that serves as the primary color, conveying trust and professionalism.
2. Striking Coral - 
                            
                            #FF4C4C
                         - A bright coral that stands out against the primary blue, perfect for call-to-action elements.
3. Light Grey - 
                            
                            #D9D9D9
                         - A soft light grey that provides a neutral background, ensuring other colors pop.
4. Medium Grey - 
                            
                            #A6A6A6
                         - A mid-tone grey for text and secondary elements, offering a contemporary feel.
5. Dark Grey - 
                            
                            #4D4D4D
                         - A deep grey that can be used for headers and footers, enhancing readability and contrast.

This palette creates a modern and professional appearance while ensuring that your CTAs grab attention.

Would you like any adjustments to the colors, or do you have specific elements in mind for your app that we should consider?