The colors you're using are the ones I employ, so I congratulate you on your good instincts.
I generally start out with a yellow ochre and indian red and white in the light flesh. As the tones get darker I add raw umber to create a darker yellow and have less white. To all mixtures I'll add a neutral gray of the same value (made from black, white and umber) to decrease intensity where appropriate. Venetian red is added where more warmth is needed. The other colors are not used in the flesh save, for reflected lights.
You can see a picture of my palette arrangement here on Beth's website:
http://www.ewschott-fineart.com/File...elsonSpec.html
I didn't want to post this picture without Beth's permission.
The above is a gross simplification of what I do. When I teach a class or workshop I demonstrate the preparation of my palette and it's practical usage for approximately twelve hours. If you think about it, color mixing consumes about half of the time you spend painting!
The bad news, as far as what you're asking here, is that my palette is able to traverse the broad range of all human complexions from albino through African American. The proportions vary according to each individual persons skin and the specific area you are painting. Once I know what value I want to mix and by asking myself if it's, more yellow or red and if it's more intense or neutral, I can arrive at the correct mixture. Unfortunately this doesn't help you out if you don't know what color to mix.
Therefore I agree with Michelle's suggestion but I would strongly suggest having this model sit for a color study from life. Don't fret the drawing in the study, just go for the color. Good luck.