Need Help with IF/ELSE Errors

Your approach is going to snap from one color to another. You have a slider for temperature, a to set a minimum value, a slider to set a maximum value, and a slider to set an average temperature. As the temperature slider approaches the minimum value, the color changes to one color; as the temperature approaches the maximum value, it changes to another color.  I'm not sure I understand what the Average slider is supposed to do. 

If I were designing this kind of graphic, I would set up three rectangles on a single shape layer, attach the sliders to that shape layer, then animate the Rectangle/Transform/Opacity of each layer using an expression on Transform Rectangle 1 (hot), and Rectangle 2 (Cold) while leaving Rectangle 3 (Normal) untouched. If the temperature slider is animated, you can establish a range of values to blend the colors as the temperature changes and use blend modes between the Rectangles to give you pleasing colors. 

Hot Opacity Expression:

t = effect("Temperature")("Slider");
tMax = effect("Max")("Slider");
av = effect("Differende")("Slider");
tMin = tMax - av;
linear (t, tMin, tMax, 0, 100)

Cold Opacity Expression:

t = effect("Temperature")("Slider");
tMin = effect("Min")("Slider");
av = effect("Differende")("Slider");
tMax = tMin + av;
linear (t, tMin, tMax, 100, 0)

You could use the same approach to directly control the color of a fill by figuring out the RGB values you want for the color changes and replacing the 0 and 100 with the starting and ending RGB colors. That would require only one shape layer and an expression for color that looked something like this:

t = effect("Temperature")("Slider");
tMin = effect("Min")("Slider");
tMax = effect("Max")("Slider");
dif = effect("Difference")("Slider");
Nor = [.2, .9, .2, 1];// Green
Hot = [.9, .9, .2, 1]; // Yellow
Cold = [.2, .5, .9, 1]; // Blue
Av = tMax - tMin;
if (t < tMin + dif)
	linear(t, tMin, tMin  + dif, Cold, Nor);
else if (t > tMin + dif && t < tMax - dif)
	Nor
else if (t > tMax - tMin)
	linear(t, tMax - dif, tMax, Nor, Hot)

Color values are always 0 to 1 for R, G, B, and A unless you use a converter method in the expression. I usually just use the float values and have my Info panel always set to decimal (32-bit), so the color picker gives you the decimal values for RGB. You'll end up with something like this:

I included a project file for you to play with.

If you want to use Hex codes and a single shape layer fill or color effect like tint, this expression should do it. I cleaned it up a bit so it's easier to understand the structure.

Hot = hexToRgb("F6A70F");// Oraange
Cold = hexToRgb("3380E6"); // Blue
Nor = hexToRgb("26D071"); // Green
t = effect("Temperature")("Slider");
tMin = effect("Min")("Slider");
tMax = effect("Max")("Slider");
dif = effect("Difference")("Slider");
Av = tMax - tMin;
if (t > tMin + dif && t < tMax - dif)
	Nor
else if (t < tMin + dif)
	linear(t, tMin, tMin  + dif, Cold, Nor);
else if (t > tMax - tMin)
	linear(t, tMax - dif, tMax, Nor, Hot)
else
	Nor

I hope this makes sense and helps a bit.