Accessing COLOR as input variable breaks shader unintuitively

**Godot version:**
3.1 Alpha 2

**OS/device including version:**
Windows 7 Professional Service Pack 1

**Issue description:**
Shader turns entire sprite white.

Tried both of the following shader coding:

```
shader_type canvas_item;

//Set target colors.
uniform vec4 target_color_1 : hint_color;
uniform vec4 target_color_2 : hint_color;
uniform vec4 target_color_3 : hint_color;

//Set replacement colors.
uniform vec4 replace_color_1 : hint_color;
uniform vec4 replace_color_2 : hint_color;
uniform vec4 replace_color_3 : hint_color;

void fragment() {
if (COLOR == target_color_1)
{
COLOR = replace_color_1
}
if (COLOR == target_color_2)
{
COLOR = replace_color_2
}
if (COLOR == target_color_3)
{
COLOR = replace_color_3
}
}
```
And:
```
shader_type canvas_item;
uniform float threshold : hint_range(0,1) = 0.01;
uniform vec4 T0:hint_color = vec4(0.058823, 0.058823, 0.058823, 1);
uniform vec4 T1:hint_color = vec4(0.254902, 0.254902, 0.254902, 1);
uniform vec4 T2:hint_color = vec4(0.45098, 0.45098, 0.45098, 1);

uniform vec4 R0 : hint_color;
uniform vec4 R1 : hint_color;
uniform vec4 R2 : hint_color;

void fragment() {
vec3 c = COLOR.rgb;

if (abs(T0.r-c.r) <= threshold && abs(T0.g-c.g) <= threshold && abs(T0.b-c.b) <= threshold) {COLOR.rgb = R0.rgb;}
if (abs(T1.r-c.r) <= threshold && abs(T1.g-c.g) <= threshold && abs(T1.b-c.b) <= threshold) {COLOR.rgb = R1.rgb;}
if (abs(T2.r-c.r) <= threshold && abs(T2.g-c.g) <= threshold && abs(T2.b-c.b) <= threshold) {COLOR.rgb = R2.rgb;}

}
```
(Ignore the green pixel.)

**Steps to reproduce:**
Run app.