Let's let
H = Hearts
D = Diamonds
C = Clubs
S = Spades
Assume order matters. For example 10H, 9H, 8H, 7H, 10C, 9C, 8C is different from 7H, 8H, 9H, 10H, 8C, 9C, 10C
Number of ways:
1st card: 26 (because there are 26 red cards)
2nd card: 24 (because there are 24 remaining possible red cards, as you can't pair with the first card)
3rd card: 22 (similar logic as above)
4th card: 20 (similar logic as above)
5th card: 26 (because there are 26 black cards)
6th card: 24 (because there are 24 remaining possible black cards, as you can't pair with the fifth card)
7th card: 22 (similar logic as above)
Multiply all together: 26*24*22*20*26*24*22