Shopping requests are off-topic here, but you're after a "colorimeter".
When it comes to measuring a display's ability to represent color, you're not likely to get "hex codes" back, at least not in the way that you're thinking (due to the sensor's dynamic range and potential for saturation that needs to be avoided).
For example, displaying
#00FF00 (pure / bright green in 24-bit color) will not likely result in a simple output value of
#00FF00 from the tool, because if the display is too bright there is no way to represent this fact - you've run out of dynamic range. A tool could provide this value against a gamut, but there will need to be other information alongside to indicate artefacts that cannot be accurately represented in 24-bits alone.
Another aspect (as touched on in the Eizo page linked below) - what happens if a display renders "green" at a wavelength that is not the agreed upon wavelength? This cannot be accurately represented with 24-bit color.
This is also emphasised by the fact that many consumer displays (e.g: smart phones) produce massively over-saturated images, along with the incoming trend for consumer displays to support HDR, with 10-bit or 12-bit color.
There are a large number of things that will make simply displaying "test patches" awkward and likely invalid.
These devices (and their accompanying software) will provide you with information on the display's ability to render a gamut - for example a coverage of 97% of sRGB is quite reasonable for modern consumer displays, with high end displays targeted at professional use (aka "wide gamut") achieving good coverage of Adobe RGB.
Without meaning to be rude, I'd suggest you read up on the topic before turning your hand to writing reviews - Eizo have a decent write up: The Ability to Display Color Correctly Is Vital: Understanding the Color Gamut of an LCD Monitor
With ΔE* you should understand that the complexities of human color vision also play their role... for example we are far more sensitive to some wavelengths (green) than others (blue / red), and this must be accounted for in the model.