When we use the expression rainbow, we are necessarily talking about the flop. If the flop is 2-3-4 all hearts, it is called a single-color flop. If the flop is 2-3-4 with 2 hearts, we say it's a flush draw flop and if the flop is 2 of spades, 3 of hearts and 4 of clubs (or diamonds), we say it's a rainbow flop. What matters is that it is 3 different colors. When we talk about color in poker, we are not talking about red and black, but hearts, diamonds, spades and clubs.
Specificity of the rainbow flop
In general, there will be less action on the rainbow flop. On a flop with a flush draw, it is possible for an aggressive player to check-raise to increase his equity in the hand. On rainbow flops, flush draws will be impossible and therefore, players will have less incentive to continue in the hand. A rainbow flop is also a synonym for a "dry flop", in other words, a flop that will generate less action than a dynamic flop such as 9 of spades, 10 of spades and jack of diamonds.