Confetti particles currently appear (visually) to not respect the provided color options. This is because there is no way to control the usage of opacity, which on different colored backgrounds appears to alter the color of the particles.

Here is a link to an example of this reproduced. Judging from the source, it appears that for this specific example the opacity is not intended to apply as it is determined by a randomly assigned radius, something that is not used in rendering square particles. Ideally the opacity would be able to be controlled by an additional option, or at the very least disabled by an option.