obtain data from a colour scaled map

i have a map that shows magnetic field strength and it uses a color scale so that when the colour is red on the map this means that the magnitude is approaching positive maximum and if its green then its close to 0 and when it gets to blue its approaching negative infinity, and i am looking for a way so that whenever i move the cursor on the map i would know the value of the magnetic field at that point based on the colour

[Adam Danz]

What version of matlab are you using and how was your map constructed?

[Ahmed Abdulla]

i am using MATLAB R2019a, the map is taken from a research paper so its just an image file