I: Mr B. Finds Pamela Writing, Joseph Highmore, 1743-4; Tate Collection N03573
Pamela had been a lady's maid to old Mrs. B for three years when her mistress died. Mr. B, her son, asked Pamela to stay on to "take care of [his] linen"; he also told the housekeeper to "give [her] mourning with the rest [of the servants]", and gave her the money that was in his mother's pocket when she died, which is later referred to as a common tradition. This scene is set in the very first letter of the book, when he interrupts her as she is writing in the late Mrs. B's dressing room to send most of the money to her parents. He reads the letter, and compliments her on its innocence and her good handwriting.
In this picture, Pamela is dressed in a black mantua - most likely the mourning clothes given to her by the housekeeper. The front edges of her bodice are laced together with a black cord, crossing twice and tying over a white kerchief. The cuffs on her wide sleeves aren't winged, but probably just turned back; her shift sleeves end in large ruffles. This gown might be linen or wool, since it is coming from the housekeeper: lady's maids might wear silk garments that were cast-offs from their mistresses, but that's not the case here.
Pamela's cap is rather fashionable, being flat and pinned on top of the head - this can be seen on wealthy ladies in portraiture and satires of the time. It's more about satisfying the requirement to wear some sort of a cap for propriety's sake than an actual attempt at covering the hair. There is a narrow ruffle around the edges, and it's trimmed with a black ribbon.
Her apron's waistband dips in the center front. It is long, to the floor or close to it when Pamela is standing, and not sheer - a working apron. Later, Mr. B gives her "three of [Mrs. B's] cambric aprons, and four holland ones": these might be translucent and/or shorter, decorative garments that, like her cap, are more about being worn to show respectability rather than for actual protection.
Pamela mentions in her letter that she's going to wrap the money in a handkerchief and put it in one of Mrs. B's pillboxes, which appears to be what's happening in the picture.
Mr. B, when he interrupts Pamela, is dressed in a blue silk coat, black breeches, and a black or very dark blue waistcoat. The buttons and buttonholes on the coat are arranged in pairs, with slight spaces in between, a design variation that can occasionally be seen in other paintings. The waistcoat, which might have a damasked pattern, descends to just above the knee; the oversized pocket flaps are lined with a white linen or silk. His white stockings are probably silk; his black shoes are difficult to make out in detail, but have no heel and a slightly squared toe. It looks like Mr. B isn't wearing a wig, but has his natural hair tied back with a dark bow.