I haven't used the BVM stuff but have used tints from tailormadedecals.com mixed into metallic paint. The colours need to be metallic otherwise it looks like paint not metal. You layer the different colours and then carefully rub down through to expose each layer. The first i.e. bottom colour is the hottest colour usually blue, work your way up to the coolest colour usually yellow and finally topcoat with the aircraft normal colour. Then rub down as required to get the colour you need. It will be fringed with the layers of cooler colours as if the heat has spread outwards from the hottest spot. The result, using 7 different heat colours is shown below on my F-100.