For those in other countries that don't have access to Craft Smart Metallic Silver. This post is for you!
While it has been proven that the particular paint used as the base for a reflective mix
does matter, I am in hopes that most flat white paints can be substituted for the Sherwin-Williams Luminous White I used. Time will tell.
Substituting another metallic silver paint for CSMS will be more difficult, but I hope not impossible. I would suggest going to your local arts and crafts store and purchasing a small bottle of every water-based metallic silver paint you can find. Put a drop or two of each of these on some white cardboard and spread them around a bit (I find that blowing on the paint is better than smearing it with a finger) being sure to label them so you know what's what; when the samples are dry, look at them under a bright light at an angle such that you see the reflectiveness of the silver particles. Look for any prismatic effect where the sparkles are all the colors of the rainbow. This effect is undesirable. If you can't find a paint that doesn't have this prismatic effect, well... you gotta do what you gotta do. Try one anyway and see what happens.
If you have to use a silver paint other than CSMS you will have to experiment with the amount used. Start adding drops of the substitute silver paint into an ounce (or half ounce) of whatever white paint you are using until you reach the shade of gray you are after. Make a sample of this mix and see what it looks like dry and under a bright light. You should be able to see the sparkles in the mix.
If you find a mix that works for you please be sure to post it on the forum!
Good Luck!