Formatting code for forms - allowing for mistakes

Hi all,

I found a useful code for changing fill and font colors based off a value.
I've got this working; e.g.

else if(event.value == "A"){
event.target.textColor = color.white
event.target.fillColor = ["RGB", 255/255, 0/255, 0/255];}

I'm trying to add code allowing for user typos/mistakes.

I added a clear form button, this works.
However, when re-entering the correct value, it doesnt detect the correct  command.
e.g

user types A (gets the above)

user deletes the A, types B but the field doesnt detect;

else if(event.value == "B"){
event.target.textColor = color.white
event.target.fillColor = ["RGB", 0/255, 255/255, 0/255];}

I tried adding the below that i found to the start but doesnt work. 

if(event.value == ""){
event.target.textColor = color.black
event.target.fillColor = color.transparent;}

The form will be a basic 5 field template, they can fill and print/stick to a file.
Variable A

VariableB

VariableC

VariableD

User Name

Full code:

if(event.value == ""){
event.target.textColor = color.black
event.target.fillColor = color.transparent;}
else if(event.value == "A"){
event.target.textColor = color.white
event.target.fillColor = ["RGB", 255/255, 0/255, 0/255];}
else if(event.value == "B"){
event.target.textColor = color.white
event.target.fillColor = ["RGB", 0/255, 255/255, 0/255];}
else if(event.value == "C"){
event.target.textColor = color.black
event.target.fillColor = ["RGB", 255/255, 255/255, 0/255];}
else if(event.value == "D"){
event.target.textColor = color.white
event.target.fillColor = ["RGB", 0/255, 185/255, 255/255];}
else if(event.value == "E"){
event.target.textColor = color.white
event.target.fillColor = ["RGB", 64/255, 64/255, 64/255];}
else if(event.value == "F"){
event.target.textColor = color.white
event.target.fillColor = ["RGB", 0/255, 255/255, 0/255];}
else if(event.value == "G"){
event.target.textColor = color.white
event.target.fillColor = ["RGB", 64/255, 0/255, 0/255];}
else if(event.value == "H"){
event.target.textColor = color.white
event.target.fillColor = ["RGB", 162/255, 255/255, 0/255];}
else if(event.value == "I"){
event.target.textColor = color.white
event.target.fillColor = ["RGB", 255/255, 110/255, 0/255];}
else if(event.value == "J"){
event.target.textColor = color.white
event.target.fillColor = ["RGB", 128/255, 0/255, 255/255];}
else if(event.value == "K"){
event.target.textColor = color.white
event.target.fillColor = ["RGB", 255/255, 0/255, 0/255];}
else if(event.value == "L"){
event.target.textColor = color.white
event.target.fillColor = ["RGB", 0/255, 255/255, 0/255];}
else if(event.value == "M"){
event.target.textColor = color.black
event.target.fillColor = ["RGB", 255/255, 255/255, 0/255];}
else if(event.value == "N"){
event.target.textColor = color.white
event.target.fillColor = ["RGB", 128/255, 0/255, 255/255];}
else if(event.value == "O"){
event.target.textColor = color.white
event.target.fillColor = ["RGB", 0/255, 185/255, 255/255];}
else if(event.value == "P"){
event.target.textColor = color.white
event.target.fillColor = ["RGB", 64/255, 64/255, 64/255];}
else if(event.value == "Q"){
event.target.textColor = color.white
event.target.fillColor = ["RGB", 0/255, 255/255, 0/255];}
else if(event.value == "R"){
event.target.textColor = color.white
event.target.fillColor = ["RGB", 255/255, 26/255, 146/255];}
else if(event.value == "S"){
event.target.textColor = color.white
event.target.fillColor = ["RGB", 64/255, 0/255, 0/255];}
else if(event.value == "T"){
event.target.textColor = color.white
event.target.fillColor = ["RGB", 255/255, 110/255, 0/255];}
else if(event.value == "U"){
event.target.textColor = color.white
event.target.fillColor = ["RGB", 0/255, 255/255, 0/255];}
else if(event.value == "V"){
event.target.textColor = color.white
event.target.fillColor = ["RGB", 0/255, 255/255, 0/255];}
else if(event.value == "W"){
event.target.textColor = color.white
event.target.fillColor = ["RGB", 0/255, 255/255, 0/255];}
else if(event.value == "X"){
event.target.textColor = color.white
event.target.fillColor = ["RGB", 0/255, 255/255, 0/255];}
else if(event.value == "Y"){
event.target.textColor = color.white
event.target.fillColor = ["RGB", 128/255, 0/255, 255/255];}
else if(event.value == "Z"){
event.target.textColor = color.white
event.target.fillColor = ["RGB", 0/255, 255/255, 0/255];}

I've attached the trial form as it stands

Thanks in advance for any help!