If you go down to the woods for an outdoor portrait with your flash, you may notice a disparity between colour temperatures in terms of unflattering skin tones and washed out greens that can often have a negative impact on outdoor portrait images and trying to fix this in post processing will take time, effort and skill.So what is affecting the images? The cause is the colour disparity between the different light temperatures. Flash is a daylight-balanced light source, with a temperature of 5500K. However, shade has a higher temperature than 5500k and can range from 6500k-9000k depending on the type of shade you place your subject in.One advantage of using flash is that we can gel it to help adjust and balance colour temperatures. The two most useful types of gels for flash are CTO’s & CTB’s, but for this blog, I will focus on the CTB.CTB (Colour Temperature Blue) CTB gels come in varying strengths, full, half, quarter. (often called cuts) By using a half CTB gel on the flash we can convert the colour temperature from 5500 to 7900k and by setting the cameras (WB) White Balance between 7100k and 7900k we can balance the ambient light temperature and the flash temperature, thus achieving a balanced look and feel to images.By using gels to adjust colour temperatures, it will equip you to remove or reduce the unflattering colour disparity often encountered when taking outdoor portraits in the shade.Personally, I am still learning about colour science in relation to flash & gels, but like most things in relation to photography, you just have to experiment and practice.