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How much blood constitutes a Reiyah? What amount of blood on colored clothing would no longer be considered a kesem but would make the woman a nidah?

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 beginning 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.