Your two kittens are probably black and their Classic tabby pattern is showing as a "ghost pattern".
ALL cats are tabby cats, carrying some variation and/or combination(s) of Abyssinian, Classic and Mackerel. What is required for their tabby pattern to be readily visible is the "Agouti" gene.
With agouti, you get vivid tabby patterns.
Without agouti, you get solid colored cats.
Some color breeds have bred agouti out of their gene-pool, like the traditional Siamese breeds, Russian Blues, Korats, Chartreaux and British Blue Shorthair. Some Siamese breeders have introduced agouti back into the breed and a new classification was created for registration/showing and they are called "Colorpoint", a registry for the 'unaccepted' Siamese colors and markings of red, tortie and lynx.
Generally, if a cat carries agouti (AA or Aa) they will be vividly and unmistakably marked. If they do not carry agouti (aa) you can sometimes see the ghost markings of their tabby pattern in their solid coat.
A side note:
Agouti affects only black-based coats by banding hair-shafts in alternating colors. All red cats, whether they carry agouti or not, will show their tabby patterns. Longhair coats will be more difficult to discern and some breeders have worked very hard to breed cats whose red coats are very uniform in color to appear solid red with little visual evidence of the tabby pattern the red cat carries.
This is also why many Black/Red Torties (no agouti) will still show their tabby pattern in their red fur.
Check out this topic discussing and showing examples of agouti, smoke and other genes that govern and affect hair-coat and pattern.