ETA: Occurs to me: is anyone round here colourblind? Because that would be a very pertinent perspective.
Why yes I am!
I am colour blind and I am an artist.
This is the form of colour blindness I have (there are several different kinds) from Wikipedia
Deuteranomaly (most common — 6% of males, 0.4% of females):
[19] Having a mutated form of the medium-wavelength (green) pigment. The medium-wavelength pigment is shifted towards the red end of the spectrum resulting in a reduction in sensitivity to the green area of the spectrum. Unlike protanomaly the intensity of colors is unchanged. This is the most common form of color blindness, making up about 6% of the male population. The deuteranomalous person is considered "green weak". For example, in the evening, dark green cars appear to be black to Deuteranomalous people. Similar to the protanomates, deuteranomates are poor at discriminating small differences in
hues in the red, orange, yellow, green region of the spectrum. They make errors in the naming of hues in this region because the hues appear somewhat shifted towards red. One very important difference between deuteranomalous individuals and protanomalous individuals is deuteranomalous individuals do
not have the loss of "brightness" problem.
Here's a rant about it:
The number one most infuriating question I recieve is "What colour does this look to you?" Usually it's something that is red or green. - I know a pine tree is green, if someone asks me what colour it is I will say green. Then they accuse me of lying.
If it's something ambiguous, I say -I don't know, or something wrong (I mistake green for purple on occasion) Then they usually laugh at me. - hahhaha you can't tell this shirt is green.

I'm so glad I could amuse you.
Next irritating question - "What does red look like to you?" There is no way to answer that question. Red to me looks like stop signs? I know they are red. I don't know what you see. I usually just reply with- "What does red look like to you?" And they get confused and drop the subject.
A simple question people ask is - if you can't tell red and green apart, how do you drive?
Simple- Every street light in the provinces I drive in has red on top and green on the bottom.
The sideways ones, I think are red on the left and green on the right. If I were driving in a place where that was relevant I would double check.
There are lots of fun assumptions about colour blindness.
I do not see in black and white.
Objects that are red are not invisible to me. I cannot see through red walls (yes people have asked me this.)
People also don't realize how unnecessary colour is to vision. If you take a picture of a red hat on a green background and remove the color- you can still see the hat unless the tones are exactly the same. Even if the tones are similar if one is smooth and the other is rough you can see the difference.
That's the trick with colour blindness tests, all the dots are tonally exactly the same, it's just the colour that's different. I can't tell the orange dots from the green dots so I can't see the number.
If it was an orange felt number on a flat green surface I could see it.
If it was a light orange on a dark green I could see that too.
If I go shopping I bring a friend. I only buy blue and black pants. My shoes are black. That way I don't go out looking like a crazy clown lady.
At the end of the day I don't miss the colours I don't see. I have never seen them. My vision has always been this way, and nothing looks odd to me.
I have always wanted to see from another person's eyes before, but in conversation it has been impossible for them to describe theirs to me. Though they do have a fun time trying. (They are always the ones to ask. I never have.)
A stop sign is a good example.
They say- red is like a stop sign.
Well I see stop signs. I knew it was red before we had the conversation. I can't see it harder, see it better?
Pink and Orange are the same- I don't care what all you people say! If someone shows me something that is pink or orange I make an educated guess and I'm usually right.
If someone asks me what colour their orange is, I punch them in the face.
Personally I despise when people ask me 'how I see' or try to help me figure colours out. I will never ever see the same way other people do.
Colour blindness is considered a disability. I cannot get a job as a pilot, driving trains, many forms of engineering.
Electrical work is dangerous and I am terrible for picking good bananas.
I have trouble with games and software text regularly and my computer is set up to compensate for that.
Colorblindness is very common within both branches of my family.
On my father's side, it's almost like a badge of honour. His philosophy is we're right and everyone else is wrong. (he doesn't care that we're out numbered)
Fun fact- camouflage doesn't work as well on colour blind people. I have an great uncle who was used as a scout in the army because camo just didn't work on him.
I havn't had a chance to test myself on that game, but would like to try...
/End rant
As to the original question
How would you describe colors to a blind person?
Bottom line, I don't see red or green correctly. This is how it usually goes.
"My shirt is green," they say.
"I mistake green and brown. Sometimes green and purple."
"Well, it's like grass."
"Yes, I know grass is green. Again, green and brown often look the same. How is green different from brown?"
Followed by a long pause. Perhaps more props will help.
"It's greener. These leaves are green, this tree trunk is brown."
"I know leaves are green. I cannot distinguish it from this brown colour here." I point to a fence the same tone as the leaves.
"Well it's just greener. Brown, I guess is redder."
"I have trouble with red. Orange and pink are the same thing."
"No pink is redder."
"I don't know what red is. How can I know what redder is?"
/continuous loop forever.