EDIT: Your not seeing double, Obviously the previous poster and I were hard at work on the same idea at the same time!
If you are used to working in monochrome or grayscale (as with graphite or charcoal) then determining the value of your colors is possibly the most difficult aspect of working in color. Believe me, it is not just beginners that struggle with determining the value of colors! I know I do!
First let me say that it is perfectly OK to try and get the color right from the beginning. You do not have to layer to reach the correst color, but in all likelyhood will end up with a few layers anyway. You will find that color has a lot of variation and gradation and that layering is necessary, but I usually try to find the general color of an area and try to get it right from the start. My layering will then include adding lighter and darker colors for lighter areas and shadows. The perhaps subtle changes in color to modify the temperature or intensity. But the closer you are to some of the colors from the beginning is a good idea, in my opinion.
One way that people try to focus in on the color without being influenced by the thing they are painting (and the preconceived notions of what color it is ) is to take a piece of paper and punch a hole in it. Then hover the hole over your ref and you'll see only the color and value without knowing what it is!
Similarly, of you have any photo manipulation computer program that has an "eyedropper" tool that lets you pick colors from the image, you can see what colors are in a certain area. You can then make rectangles or swatches if you want - as a guide. Something like this.
Hopefully these types of swatches will help you choose a pastel of a similar value and color. I sometimes print them out and make pastel marks right on the print to compare.
You'll notice that only one of the swatches of the dog comes even close to being black - all the others are lighter shades of grayish color. You'll notice one of them is rather greenish, probably getting some green color reflecting off the grass. Something to think about when doing the background colors! Lot's of people like to change background colors in photos, but those background colors influence the colors on the subject sometimes.
I often show these types of swatches as a good tool to see the colors/values better. But I always urge people not to assume that this means that they have to use the exact colors of the photo. First, photos don't really match the colors of reality. Second, you should choose colors that you think work best in the painting! No one will see the photo when your painting is hanging in a gallery or someone's house! These swatches should be seen as just a very general guide!
Hope this helps!