To paint blonde hair, you might want to just paint it yellow, and you would be mistaken. Some blonde hair, in fact has little to no yellow. Regardless, you're going to want to mix your yellow with purple to tone it down.
You'll also notice, in this reference photo, that while this woman's hair is pretty light, there are some very dark parts to it too. Getting that variation in is important. Now that I'm looking at it again, I even see a little bit of blue in this woman's hair. It's subtle, but it's there.
Note: Okay, I realize this model's hair is probably dyed, as evidenced by her dark eyebrows, but I really like the shade. Beyond that, I love the angle she's at and the position her eyes are looking, so I really wanted to use this photo.
The first thing I did before painting this was wet the paper. That way, I could utilize the water on the paper, as well as the water I mixed into the paint on the palette, to get the super light color I was going for.
After I got the base tone down, I dipped my brush back into the same yellow mixture, and with less water mixed in it now so it would be darker, I painted "lowlights" or parts of the hair that are darker than the base. This helped give the hair texture.
Remember I said I noticed some blue in it? Well, I mixed some black into the prussian blue that was already on my palette. I was going to use this, until I remembered a very important rule when painting portraits. When using bright colors, blue, red, etc, always mix the color with it's complement. You want to use muted versions of these colors. So I mixed a bit of orange into the blue. Then I used a filbert brush to paint streaks of it on top of the yellow. I did all of this with the surface wet, so my edges would be soft.
If you were doing this in oils or acrylics, the process would be pretty much the same, except you would use white to lighten and blend out your edges with your brush as you went, instead of relying on the water to do the lightening and blending for you.
When I looked at the painting again and compared it to my reference photo, I noticed that I painted one of the blue streaks too thick. Now, watercolor isn't easy to paint over, but I'm going to try layering over the blue with orange to neutralize it(which I did) and then painting over that with my yellow mixture.
I also noticed that the yellow overall was too bright, so I layered more purple over it.
Even in this woman's very light hair, there were some parts that were actually pretty dark. I mixed the blue that I'd used for the blue highlights into some burnt umber and then mixed that into the same yellow mixed with purple and, using my smallest filbert brush, painted on the streaks that you see.
I was careful to make sure the streaks were the right shape, which is tapered at the top and getting thicker going down, and at least approximately in the right places. If you just put in streaks randomly, it won't work.
Here's my painting, "Kate Moss(2005)". Doing this painting was actually the first time it occured to me to mix purple into the yellow to make it more natural looking.
But you can see for her under layers I used burnt sienna, which is pretty dark compared to the yellow.
You can watch this process in action in the video below.
Here are Amazon Affiliate links for the supplies I used for this painting. If you buy from these links, I get a small percentage at no extra cost to you.
Amazon affiliate links
Winsor and Newton Cotman watercolors
Royal Talens Van Gogh watercolors
Fabriano Studio 140 pound cold press watercolor paper
Painter of portraits and wildlife