When we use the expression rainbow, we are necessarily talking about the flop. If the flop gives 2-3-4 all in the heart, we will say that it is a monochrome flop. If the flop is 2-3-4 with 2 hearts, we will say that it is a flop with color print and if the flop is 2 of spades, 3 of hearts and 4 of clubs (or diamonds), we will say that the flop is rainbow. 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 about heart, 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 make an check-raise to increase his equity in the hand. On the rainbow flop, color draws will be impossible and therefore players will have less incentive to continue in the hand. A rainbow flop is also synonymous with "dry flop", in other words, a flop will generate less action than a dynamic flop such as 9 of spades, 10 of spades and jack of diamonds.