She saw the tuhr then she had some discharge – what should she do?

We do not know what you mean by “a little bit of white
discharge was coming out.” If what you mean is that you saw the white
discharge which is the sign of purity (tuhr), then whatever yellowish or
reddish discharge comes out after that is not regarded as menses, because
Umm ‘Atiyyah (may Allaah be pleased with her) said: “We did not regard the
brownish or yellowish discharge after the tuhr as being anything that
mattered.” Narrated by Abu Dawood, 307; classed as saheeh by al-Albaani in
Saheeh Abi Dawood. 

Based on this, your
prayer is valid, and there is nothing wrong with your having had
intercourse, because you were not menstruating. 

But if what you mean is that you saw that the discharge was
still yellowish or reddish, this means that the menses had not yet ended. A
woman should not be hasty in deciding that the period has ended when she
sees yellowish or reddish discharge, no matter how little it is. The women
used to send pieces of cloth to ‘Aa’ishah with yellowish discharge on them,
and she would say, “Do not be hasty until you see the white discharge.”
Narrated by Maalik, 130. 

Pieces of cloth refers to what women use to check whether
there is any trace of menses left or not. 

See also question no.
66062. 

Based on this, then you have to make up that day, because it
is not correct to fast when one is menstruating. 

With regard to intercourse, there is no sin on you in sha
Allaah, because you thought that your period had ended, and you did not
deliberately commit a haraam action. Allaah says (interpretation of the
meaning):

 “And there is no sin on you concerning that in which you
made a mistake, except in regard to what your hearts deliberately intend”

[al-Ahzaab 33:5].