Never. The only person in ivory should be the bride. It is inappropriate for a person at a wedding, mother or not, to wear the same color as the bride.
The Mother of the Bride CAN wear black---but it would be very poor form. People at weddings should be wary of black.
Saree
No, the mother of the bride does not have to wear a blue dress. The mother of the bride can wear any color dress she likes best, provided it's not white to upstage the bride.
the mother of the bride should wear red because it color coordinates well.
Usually, the mother of the bride tends to wear colors that compliment the bridal party rather then match them. Colors that complement the bridal party are for example, if the colors are blue, the mother of the bride could perhaps wear royal blue, baby blue, navy, ocean blue. I think you get the idea. Usually there are two colors for the wedding and so that would open up more choices for colors to wear for the mother of the bride or groom.
pale blue
White or the color dress of the bride
No, it is not traditional for the mother of the bride to buy a nightgown and pajamas for the bride and groom. Sometimes the groom may wear just his underwear; just wear pajama bottoms or even be in the buff. The bride should pick out a nice negligee for her wedding night.
The general etiquette for the mother of the groom is to wear an elegant dress in a neutral color, such as beige or gray, so as not to outshine the mother of the bride (regardless of the color of the mother of the bride's dress).
The mother of the bride should not wear white but perhaps a cream color or a nude color. Navy blue is also very nice but nothing with loud patterns or big bows.
The mother of the groom should ask the bride's mother what color she will be wearing. Or ask the bride what colors will be used for her bridemaids. There are only two colors she shouldn't wear - white or black. White, because only the bride should wear white. Black because it means mourning and women shouldn't be wearing black for a happy ocaision. Most mother wear blue. Although in these modern times of looser protocols, the mother of the bride and groom can choose to wear black if the color theme of the wedding is black/white. Ivory, cream, off white could then be considered the "white" colors to wear without necessarily wearing white.