Hi,
A quick tip I was given on here is to stir your 2 gels and then take a small blob of each gel using different cocktail sticks and place it on a ceramic tile (handsdown plastic backed wipes are really good as well). Mix the 2 colours together to see if you like the colour.
You may want to add another blob of one of the colours to get a better shade. Just remember what proportions you add e.g. 1 blob of colour 1 and 2 blobs of colour 2 , so that when you do your final mix you get the same proportions.
If the colour is too dark consider adding either white or another pale colour. If it is too light add black or another darker shade.
Pop on to the Bio Sculpture group for some ideas on mixes and layering we have already tried.