Specialized clothes for riding have been around for a very long time.  For reasons of cleanliness and comfort specialty clothing was the rule of the day for most wealthy people. 

Riding horses is an athletic endeavor, for both the horse and the rider, so riding clothes would expect to get sweaty, muddy, and any grease or oil that was used on the leather would rub onto the clothing.  The romantic idea that fair lady rode to the ball in her ballgown is simply not true.  She might have carried her gown with her and changed, or more likely she would have taken a carriage.

Dark colors have been the norm for riding clothes for the last 100 years or so, but historically habits have been all sorts of colors. 

The riding habit quickly became the "sportswear" of the woman.  She would have worn it travelling, for outdoor activities, and as casual wear. 

Like the hemline of today, riding skirts and habits have varied up and down over the years.  At one point the skirts were so long that they hung down halfway to the ground, and sometimes the horses would get their legs tangled in them.  Currently, it is usual for the hem to be st or just above the riders ankle when she is mounted, showing the boot, stirrup and spur (if worn).

It is likely that women have always worn some type of pants under their riding habit, large pantaloons or petticoats are uncomfortable in the saddle and lead to blisters and chafing.  They would also add greatly to the laundry burden.  Early riding pants for women were probably made from Chamois, or cloth, as men's were.

Calf buttons on sidesaddle pants are on the front of the legs, unlike astride pants which have the buttons on the outside of the calf.  This removes the buttons as a source of pressure points on the upper leg and saves the saddle from scratches.

A sturdy leather ankle boot, or perhaps a calf boot would have been worn.

Gloves are also always worn to protect a ladies hands from the reins and horse dirt.  Following both Victorian convention and practicality, black gloves are not worn by ladies unless in mourning (and riding is not a mourning activity).  Black leather dye also frequently runs when wet and would stain the hands.  So even if the habit is black, the gloves are not.

Fox hunters and ladies in appointment classes also carry a pair of rain gloves under the offside flap, usually cream or white cord or "string".  Leather reins and leather gloves can be very slippery when wet.