All these charts use Humbrol paints as the bench-mark, so if you want to convert from Heller to Vallejo then you'll have to go via Humbrol.
I have tried to be as accurate as possible but I have to point out that the conversions and the colours used in the charts are to be used as a guide only as it is impossible to be totally accurate for several reasons mainly because monitors, printers and the good old fashioned human eye vary so much. Anyway, I hope these prove useful to you.
Click on the logo for the brand you wish to view (Revell, for example) and you'll see a two charts, one converting from Revell to Humbrol and one converting from Humbrol to Revell. If the image is too small then just click on it to zoom in.
Originally this chart was requested by a colour blind gentleman who had trouble selecting the nearest colours within the greens. After a little research I found that a large proportion of modellers have some degree of colour blindness and so this chart became more and more important to finish. Although I'm not colour blind myself, I still find this to be a very useful chart.