OK -- second try:
I find a gray card to be indispensable for accurate rendition of both color and value of difficult flower color.
Gray card value is what meters are calibrated to see. It's like as if you could take all the values (lightness/darkness) of a scene and pour them into a jar and shake it up. The value that results in an "average" scene is a middle gray, the value of a gray card. If that scene's value turns out to be lighter than a middle gray (like a bright snow scene), the meter still sees it as gray and the result will be underexposure. Conversely, if you take the values of a black cat in a coal bin and average them, they will be darker than a middle gray, and the result will be overexposure because the meter thinks they should look middle gray. So the point here is that if you use a gray card to meter from in that scene, the meter sees that value and will render the scene correctly.
So I will take a gray card, place it behind the difficult flower, take a reading with the shutter button depressed slightly, holding it there while I take away the card and then depress the shutter all the way. Most cameras should allow you to do this, or they will have some way to "lock" the exposure.
The gray card doesn't really have anything to do with rendering color, as such. But if you get the value right, chances are better than the color will also be right.
So how do you get the color right? That is a matter of the color sensors in a digital camera. Some are better than others, but most have the ability to set the "white balance." In the days of film photography, you had to match the kind of film you wanted to use to the kind of light you were taking the photo under. With digital photography, you set this with "white balance." It really doesn't matter whether you use a white card or a gray card to do this. The point is that it must be a neutral value.
Someone mentioned they underexpose a bit when photographing red. I suspect this works because the meter is not quite calibrated, or it's seeing red as a dark value and so is overexposing it. Using a gray card should help in this situation. That's assuming the meter is accurate. When I used to teach photography technology, calibrating our meters was among the first things we did. It was interesting: most meters were pretty close, but some were "off" a stop or more.
I find that it's not so much the quantity of light as the quality. A good tripod goes a long way to enable the photographer to use longer exposures to make up for light quantity. For quality, that's why I like to use daylight but not direct sun. Direct sun tends to make shadows and highlights that are too far apart for a good rendition. It does make for some nice and dramatic photos, though (think Rembrandt). Some of the light sources that we use for growing orchids don't make for very good photos, even with white balance.
