You can achieve this by clicking the little plus symbol next to the measure in the chart properties, click on background color and then add a formula in the definition box similar to below.
if(RES_MICRO_SUSC_group = 'S', argb(194, 155, 244, 155), //Green
if(RES_MICRO_SUSC_group = 'I', argb(194, 244, 244, 155), //Yellow
if(RES_MICRO_SUSC_group = 'U', argb(194, 244, 155, 244), //Purple
if(RES_MICRO_SUSC_group = 'R', argb(194, 244, 155, 155), //Red
I'm basing mine on text values, but you can use numerical values and also between if you need a range.
If you know that your max value is going to be 25000 then you could define the green to be when the measure is >= 24000 then define bands of values to change the colours accordingly. you can define as many shades as you like between the highest green and the lowest red by experimenting with the argb values they are defined as this argb(Saturatiion Value, Red Value, Green Value Blue Value) all the values can be between 0 and 255.