Colorimeters -- manufactured by three different companies -- were designed to replace or perhaps enhance color-grading master stones by using a fiber-optic light system to digitally measure the color of a diamond. The accuracy is claimed to be within a half a grade of the actual “true” color. Prices have dropped dramatically over the years and will continue to do so as popularity increases and manufacturing costs decrease. At this writing, the price of these instruments averages about $5000.
It is this author’s opinion to wait for prices to come down further and for accuracy to improve. If you do choose to purchase a colorimeter, always double check it with master stones, since electronic devices are not fool proof. Your eyes, good light, master stones, and experience, will probably remain your best bet at this writing.