Sometimes a woman is bleeding but is not having a clear period. What amount of blood (on colored clothing) would no longer be considered a kesem but would make the woman a nidah through a presumed Reiyah?
Any small amount of uterine blood at all that leaves the uterus is enough. There is no minimum. (See Gemara Niddah 40a; Yoreh Deah 183:1, with Shach:3)
"...even if she only saw a drop of blood the size of a mustard seed; she must observe seven clean days for it."
On the one hand, there is no amount mentioned by Chazal, that limits the exception on colored clothing. Any amount is merely a "kesem" (Rabbinical "stain" subject to those laws)
On the other hand, some factors that would need the guidance of a Rabbi to see if a "large amount" of blood found upon colored clothing renders her a niddah or at least creates a doubt:
a) It was an amount that equaled the usual stains she experiences during an onset of a menstrual flow.
b) It happened at the time of her usual monthly period.
c) There is no way the blood can be attributed to something else.
d) She had a "Hargashah" or definite feeling of a flow from the uterus, or opening of the uterus.