Does Siteimprove check color contrast in Accessibility?

Yes, Siteimprove checks for color contrast issues in Accessibility using guidelines described below. 

The color contrast ratio required depends on the level of conformance you wish to reach. Level AAA conformance is more strict than AA.

• AA Guideline 1.4.3 Contrast (Minimum): The contrast ratio should be at least 4.5:1 for normal text and 3:1 for large text.
• AAA Guideline 1.4.6 Contrast (Enhanced): The contrast ratio should be at least 7:1 for normal text and 4.5:1 for large text.

Insufficient contrast between text and its background can create difficulty for visually impaired or color-blind users. In general, sufficient contrast is good practice as insufficient contrast can affect all users leading to important text being overlooked.

How Semi-automated checks can help with color contrast issues

Automation is not always able to detect the color contrast ratio between text and background. For instance in cases where there is text on top of an image. Therefore we have a semi-automated check called: "Is there sufficient contrast between the text and the background?" - This semi-automated check will help users confirm if there is an issue, i.e. in circumstances where color contrast cannot be determined looking only at the code.

Note: Contrast measurements are not relevant where content is hidden. However, the contrast ratio should be sufficient for hidden content that becomes visible when receiving focus.