Coloor of fried onions play important role ...dark fried onions always give dark colour to curry...onions should be golden in colour for perfect salan colour and also the colour of Red Chilly powder is equally responsible for the colour.
^ I was going to suggest the same. Try adding yogurt to it :)
Some people use prefried onions in their cooking. Try using fresh onions....more tomatoes........and make sure that your masala is of good quality. Have you tried using tomato paste in your cooking? It's faster than using fresh tomatoes......it's still healthy.........and it gives the cooking a more red color.