Color combinations that fail WCAG 2 AA 4.5:1 contrast ratio
Doubt that all of these will be tackled, but for reference (following https://github.com/twbs/bootstrap/pull/25123 which at least fixed one of the badly borked ones):
From http://getbootstrap.com/docs/4.0/getting-started/theming/
Blue
Foreground: #FFFFFF
Background: #007BFF
The contrast ratio is: 4.0:1
Text failed at Level AA
Indigo
Foreground: #FFFFFF
Background: #6610F2
The contrast ratio is: 7.2:1
Text passed at Level AA
Purple
Foreground: #FFFFFF
Background: #6F42C1
The contrast ratio is: 6.5:1
Text passed at Level AA
Pink
Foreground: #FFFFFF
Background: #E83E8C
The contrast ratio is: 3.8:1
Text failed at Level AA
Red
Foreground: #FFFFFF
Background: #DC3545
The contrast ratio is: 4.5:1
Text passed at Level AA
Orange
Foreground: #212529
Background: #FD7E14
The contrast ratio is: 6.0:1
Text passed at Level AA
Yellow
Foreground: #212529
Background: #FFC107
The contrast ratio is: 9.5:1
Text passed at Level AA
Green
Foreground: #FFFFFF
Background: #28A745
The contrast ratio is: 3.1:1
Text failed at Level AA
Teal
Foreground: #FFFFFF
Background: #20C997
The contrast ratio is: 2.1:1
Text failed at Level AA
Cyan
Foreground: #FFFFFF
Background: #17A2B8
The contrast ratio is: 3.0:1
Text failed at Level AA
From http://getbootstrap.com/docs/4.0/utilities/colors/
.text-primary
Foreground: #007BFF
Background: #FFFFFF
The contrast ratio is: 4.0:1
Text failed at Level AA
.text-secondary
Foreground: #6C757D
Background: #FFFFFF
The contrast ratio is: 4.7:1
Text passed at Level AA
.text-success
Foreground: #28A745
Background: #FFFFFF
The contrast ratio is: 3.1:1
Text failed at Level AA
.text-danger
Foreground: #DC3545
Background: #FFFFFF
The contrast ratio is: 4.5:1
Text passed at Level AA
.text-warning
Foreground: #FFC107
Background: #FFFFFF
The contrast ratio is: 1.6:1
Text failed at Level AA
.text-info
Foreground: #17A2B8
Background: #FFFFFF
The contrast ratio is: 3.0:1
Text failed at Level AA
.text-light
Foreground: #F8F9FA
Background: #343A40
The contrast ratio is: 10.9:1
Text passed at Level AA
.text-dark
Foreground: #343A40
Background: #FFFFFF
The contrast ratio is: 11.5:1
Text passed at Level AA
.text-muted
Foreground: #6C757D
Background: #FFFFFF
The contrast ratio is: 4.7:1
Text passed at Level AA
.text-white
Foreground: #FFFFFF
Background: #343A40
The contrast ratio is: 11.5:1
Text passed at Level AA
.bg-primary
Foreground: #FFFFFF
Background: #007BFF
The contrast ratio is: 4.0:1
Text failed at Level AA
.bg-secondary
Foreground: #FFFFFF
Background: #6C757D
The contrast ratio is: 4.7:1
Text passed at Level AA
.bg-success
Foreground: #FFFFFF
Background: #28A745
The contrast ratio is: 3.1:1
Text failed at Level AA
.bg-danger
Foreground: #FFFFFF
Background: #DC3545
The contrast ratio is: 4.5:1
Text passed at Level AA
.bg-warning
Foreground: #343A40
Background: #FFC107
The contrast ratio is: 7.1:1
Text passed at Level AA
.bg-info
Foreground: #FFFFFF
Background: #17A2B8
The contrast ratio is: 3.0:1
Text failed at Level AA
.bg-dark
Foreground: #FFFFFF
Background: #343A40
The contrast ratio is: 11.5:1
Text passed at Level AA
.bg-white
Foreground: #343A40
Background: #FFFFFF
The contrast ratio is: 11.5:1
Text passed at Level AA