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Linda,
As long as the surface is dry, you can glaze over it with a transparent glaze. The glaze will make the passage darker and more luminous. I used to use a medium to glaze made of damar varnish, stand oil and turpentine, but lately have been using Gamblin's Galkyd lite and also his NeoMegilp. The lite is better for glazing as the Neo I find too thick. My old formula had very strong fumes and I am trying to not use the turpentine. It also attracted every dog hair and speck of dust in the house (like a vacuum cleaner). I find that the Alkyd mediums do not do that.
One can glaze without color as a final glaze over a dry painting. I love to glaze and like the effect it produces. Look at Karin Wells work, she is the Glaze Queen and much more experienced at it than I. I am sure she has much to add to what I mentioned.
Be careful though, some of the earth colors like burnt umber leave little specks of undissolved mineral deposits. It depends on the brand. Alizarin, Ultramarine blue and violet and Sap green are a few favorite glazing colors.
On his web site Gamblin has listed the transparent colors preferred for glazing. Hope this helps.
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