What are the romance parts in super sentai series?

Why we don't have a black or a green sixth member ?

Umm...Dragon Ranger not ringing any bells for you?
And without getting too bogged down in who's a sixth and who's an extra hero, Bull Black?

I mean,why we don't have more green and black sixth members ?

Because Black and Green are usually colors used by members of the main team.

But rarely at the same time. If black is on the team, green could be sixth, and vice versa.

One probability could always be that black and green just arent as striking colors as Gold or Silver. They always stand out on the shelf in the toy store which gets the attention of children.

I think they're still too closely associated with main team members though, which I guess makes the designers hesitate to use them for 6th members. It's a thing to ask a designer personally though, they might be the only ones able to give a satisfying answer to that.

I think there should be more black and green sixth rangers because with gold and silver,it makes them stand out too much, but with black and green they will look more unified with the team.

King Ranger and Dragon Ranger would like to have a word with you.

And usually they want a color that really pops against the main team for the sixth ranger. That is why gold was the sixth for Kyoryuger rather than cyan or grey. Ranger made sense in ToQGer and gold stands out with Ninninger. Black would have been awesome, but kids like bright, shiny things (look at all the colorful explosions in sentai) and bright colors will continue to be used.

Now I would love for the 40th sentai to not use gold or silver unless it is a trim color for the main team. I think it is time for something different.