Replace all Color(hex:) references with Color constants

[joshuatbrown]

SwiftGen generates color constants based on our colors defined in Colors.xcassets. We should only use the color constants and never use Color(hex:). Let's replace all instances with color constants, and if there are no constants, create new colors in Colors.xcassets. Let's also replace any uses of Color(red:green:blue:opacity:) and Color(white:opacity:).

See #733 for more context (this is essentially a dupe of that).

[lingoslinger]

I will take this issue as I have already done the work in 1374 to move colors to their own asset catalog. Please assign to me when convenient.

[joshuatbrown]

@lingoslinger thanks! I've assigned it to you.

[lingoslinger]

Some questions as I begin work on this issue:

• What is the established naming convention for colors inthe Colors asset catalog?
• do you want other non-asset-catalog instances of Color replaced (Color(red, green, blue. opacity), Color(white, opacity), etc)?

[joshuatbrown]

@lingoslinger the goal is to use semantic naming for the colors -- see the ones that are already there for examples, like action-primary-background. And yeah, I think replacing the other Color inits is a good idea, too.

[joshuatbrown]

@setch-l for UAT, we'll just want to make sure the colors are still the same. Let us know if any color in the app seems off.