Compatibility is the primary concern when adding tints to varnishes. Not all tints or stains are compatible with varnishes and their components. Always check with the varnish manufacturer if a specific tint or stain is compatible before attempting to play chemist.
If possible, it is always recommended to stain the wood color underneath instead of coloring the varnish. Adding any colorants to varnish will add a sense of opacity to the finish itself, reducing the depth of the wood grain with each coat. When a stain is applied directly to the wood, it adjusts the tone of the wood, but the varnish will enhance the wood grain tonality even more.