I've used a photo-editing program to remove the color from the photo so it contains shades of gray only. This clearly shows how close some of your color choices are in value or tone.
The skin tones blend together into one value, whereas you'd want at least three (light, medium, dark) to create a sense of three-dimensional form. Notice also how dark the shadow under the legs is, but there's not a enough dark value on the undersides of the legs leading into this shadow. The two colors on the swimsuit also blend into one dark tone which is fine because the small fold at the waist is a darker tone, giving a sense of form.
I'm afraid there's not "quick fix" when it comes to selecting colors with the right values, it's a question of spending some time learning to associating X color with Y tone. The good news is that, with time and experience, it becomes instinctive.
The first step to solving this problem is to spend some time creating a value chart of skin tones from the colors you use. Do it for all the colors you'd typically use for skin tones. Then when you're painting and you want a light value, for instance, you consult the chart and know exactly what the color is you need to use. It's a rather a methodical approach, but with time the knowledge will become instinctive. (Ideally you'd do it for every color you use, but realistically it's too time consuming and few people do.)
The second step is to simply for subject into five values only and doing a gray-scale value study before you tackle the "real" painting. Start by blocking in the medium tone, then the dark, then the light. Then refine it by putting in a tone between your medium and light, and another between your medium and dark. (You can take it further and put in another two tones, but I think five works just fine.) Look at it again and rework the lightest and darkest tone if needed.Now paint up a value scale with your five grays from your study, then find the equivalent tones in your skin colors and paint up a chart of these five "colored values". Paint the study again using those five skin values only. Use the same grays chart to judge the values of the colors you select for the other elements in the painting, such as clothing or hair. Also, don't forget that the color of the paper can serve as one of your five tones, rather than as the background color.
Another approach to consider is to reduce the number of colors you're using, whether to monochrome (see these examples) or a limited palette (see example). Fewer colors means fewer chances of getting a value wrong.