Environment
Provide version numbers for the following components (information can be retrieved by running tns info in your project folder or by inspecting the package.json of the project):

• CLI: 6.7.4
• Cross-platform modules:6.6.0
• Android Runtime:6.5.1
• iOS Runtime:6.5.1
• Plugin(s):

Describe the bug
While trying to apply a css property for a button when its disabled i get this
Error: Failed to apply property [background-color] with value [grey] to Button(105). ZoneAwareError(file: node_modules/@nativescript/angular/zone-js/dist/zone-nativescript.js:1298:0)
at Color(file: node_modules/@nativescript/core/color/color-common.js:28:0)
at Color(file: node_modules/@nativescript/core/color/color.ios.js:6:0)
at valueConverter(file: node_modules/@nativescript/core/ui/styling/style-properties.js:613:81)
at set(file: node_modules/@nativescript/core/ui/core/properties/properties.js:670:0)
at file: node_modules/@nativescript/core/ui/styling/style-scope.js:499:0
at file: node_modules/@nativescript/core/ui/styling/style-scope.js:393:0
at file: node_modules/@nativescript/core/ui/core/view-base/view-base.js:284:0
at file: node_modules/@nativescript/core/ui/styling/style-scope.js:391:0
at file: node_modules/@nativescript/core/ui/styling/style-scope.js:340:57
at file: node_modules/@nativescript/core/data/observable/observable.js:110:0
at file: node_modules/@nativescript/core/ui/core/view-base/view-base.js:324:0
at file: node_modules/@nativescript/core/ui/core/view-base/view-base.js:341:0
at file: node_modules/@nativescript/core/ui/core/view-base/view-base.js:616:0
at valueChanged(file: node_modules/@nativescript/core/ui/core/view/view-common.js:1044:0)
at file: node_modules/@nativescript/core/ui/core/properties/properties.js:166:0
at file: node_modules/@nativescript/angular/view-util.js:290:0
at file: node_modules/@nativescript/angular/view-util.js:263:0
at file: node_modules/@nativescript/angular/renderer.js:181:0
at file: node_modules/@angular/core/fesm5/core.js:30484:0
at setElementProperty(file: node_modules/@angular/core/fesm5/core.js:28555:0)
at checkAndUpdateElementValue(file: node_modules/@angular/core/fesm5/core.js:28506:0)
at checkAndU<…>

To Reproduce
Add a styling for a disabled property and then load that disabled property
Expected behavior
No errors
Button:disabled { background: lightgray; background-color: grey; border-color: white; }