Conditional formatting based on text input in dropdown box

You can do it like this:

var L = this.getField("LPreparation").valueAsString;
var C = this.getField("CPreparation").valueAsString;
var R = L+C;
if(R=="A1"||R=="B1"||R=="B2"||R=="C2"||R=="D3"||R=="D4"||R=="E3"||R=="E4"||R=="E5"){
event.target.fillColor = color.yellow;
event.value = "M";}
else if(R=="C1"||R=="D1"||R=="D2"||R=="E1"||R=="E2"){
event.target.fillColor = color.green;
event.value = "L";}
else if(R=="A2"||R=="A3"||R=="B3"||R=="B4"||R=="C3"||R=="C4"||R=="D5"){
event.target.fillColor = color.red;
event.value = "H";}
else if(R=="A4"||R=="A5"||R=="B5"||R=="C5"){
event.target.fillColor = color.red;
event.value = "VH";}
else{
event.target.fillColor = color.white;
event.value = "";}

To see color, you will have to turn off fields highlight in Edit⇾Preferences⇾Forms and uncheck 'Show border hover color for fields'. If other users use the file, they will have to do the same on their software also.

Or since it's a calculated field you can set it to read only, that will remove highlight also.