Unfortunately, all the paint converter apps you'll find online are fundamentally flawed. This is because they're all relying on digital color swatches published on paint manufacturer's web sites. They typically compare the RGB or CMYK values of these swatches and determine which colors are closer than others.

The problem is the colors displayed online are in many cases not even close to being a match to the actual physical paint color. Even on the manufacturer's own site. So the whole thing is inaccurate from the start.